It's not their current brewery. When they merged with John Thompson & Son in 1898, they moved to the Albion brewery. Presumably because that was larger.
"After this, we returned to the brewery yard to commence our inspection of the western side of the premises. In front of the brewhouse, in a conspicuous position over the archway, a handsome clock marks the time, not only for the employes, but serves as a monitor to the inhabitants of the vicinity. At the right of the building, a stone staircase, erected in a lofty circular tower, gives admittance to the various floors of the brewhouse, by which we ascended to reach the malt stores and malt receiving room. It is in the third floor of the left wing, and capable of holding 150 quarters of malt. The mill hopper rises from the centre of the floor and underneath it there is a screen for Gleaning the malt. Passing down a steep stair we came to the mill room containing the usual machinery and a pair of steel rollers capable of grinding twenty quarters per hour. The bruised malt falls therefrom on to an elevator by which it is conveyed to the grist hopper in the next building. As we passed along to the grist room we noticed extincteur and hand grenades on shelves for use in case of fire. Under the mill room there is an engineers' and fitters' shop, and an engine house which we afterwards visited. After inspecting the grist hopper and a metal liquor back, heated by steam copper coils and erected on a gallery, we entered the mashing stage occupying the floor over the archway. Here we were shown a thirty-quarter mash tun solidly constructed of oak, containing sparger, rakes and slotted iron draining plates. This tun is commanded by a Steel's mashing machine, and the grains are emptied from it by means of a valve trap leading to a spout in the yard. On this floor there is a large space reserved for a second mash tun, and under the stage there is a sub-floor, about 5 feet high leading to a balcony in the yard, used for setting the taps of the mash tun, and where is situated the excise office. Crossing this floor, and descending a few steps, we entered the next building, where is situated the copper stage, by far the most important place in the brewery. It is floored throughout with iron plates, is very lofty and open from floor to ceiling. It contains two splendid coppers, each with a capacity of sixty barrels, and over them the underback, which is heated by steam copper coils. Here also is the hop-press, and on the ground, opposite the copper hearth, a forty-barrel copper hop-back. The ale-wort is pumped by a centrifugal pump, from this latter vessel, to the coolers in the opposite wing of the brewhouse, and thither we next bent our steps.
Ascending to the third storey, we inspected the two open coolers and two vertical refrigerators, and then descended to the fermenting room underneath. It contains seven fermenting squares the largest of which holds eighty barrels, and several other vessels. The ground floor is used as a fining house, for the firm brew porter as well as ale, when required for supplying their own public-houses. Again crossing the yard, we entered a detached building nearly 100 feet in length, the upper storey of which is devoted to the union rooms, and contains eighty union casks fitted with attemperators and overhead yeast troughs. The entire ground floor is utilized as racking rooms and ale cellars, and will hold nearly 1,000 casks. In the centre of the floor there is a racking vat holding eighty barrels, and, beyond, a yeast press with small engine attached for working the steam pump and pressing apparatus."
"Noted Breweries of Great Britain and Ireland, vol. 2", Alfred Barnard, 1889, pages 423 - 424.
This wasn't a huge brewery. The mash tun had a capacity of 30 quarters. That's only ten times the size of the one in the model brewery at Allsopp. Enough to brew about 120 barrels of beer at a time. The union room capacity seems small in relation. Assuming the union casks were the normal 61-gallon size, that's a total of 135 barrels. Or just about a single brew. Which implies that only a small percentage of their beer went through the unions.
Fermenting vessels. I always like to mention them. In this case there were seven squares, the largest holding 80 barrels. Burton brewers were divided in their preference between rounds and squares. One thing there were in agreement about: the small size of fermenters. Most of Allsopp's rounds only held 15 barrels, a ridiculously small size for the quantity of beer they were producing. Bass's squares were a bit bigger at 45 barrels, but still relatively tiny. That makes Marston's largest square quite big for Burton.
This bit confuses me: "The ground floor is used as a fining house, for the firm brew porter as well as ale, when required for supplying their own public-houses." That implies that you didn't need to fine Pale Ale, just Porter. I know that a proper Stock Pale Ale was supposed to drop spontaneously bright, if left to mature long enough. The quote also tells us that Burton brewers, just like London ones, brewed beers outside their area of specialisation to be able to offer their pubs a full range.
In 1905 Marston & Thompson merged with another Burton brewery, Evershed. I'm not telling you this for the benefit of historical precision. But because I've a little information about their beers from 1891 - 1892. I'll be sharing it with you soon.