This gown was made for the Production Elizabeth R in 1971, and was used again for a documentary on Elizabeth, in which it was greatly remade to resemble a gown worn by Princess Elizabeth in a portrait painted by William Scrots in 1546. A brooch was added, along with many pearls along the neckline. The undersleeves were removed and replaced with a pair from Elizabeth's white coronation gown from Elizabeth R. It appears, however, that the last time the gown was used in The Twisted Tale of Bloody Mary, the original sleeves were placed back on again.
This gown recently went up for auction at Bonhams. You can find the page for it here. I've also saved an image of the auction here. The page allows for you to zoom in and see the details of the dress.
It's described as "A rust coloured gown, of patterned velvet, edged with gold braid and pearl and jewel effect beads, having large turned back cuffs, revealing watered silk lining, and further lace sleeves, with boned bodice and laced back, together with a kirtle forepart of brown silk effect fabric with orange embroidered design" and is estimated to sell for: £450 - 650