OK, the Theakstons website says this:
"Old Peculier is a beautiful, yet very simple beer, brewed using a very generous blend of finest pale, crystal and roasted barley with two bitter hops combined with the majestic and noble âFuggle' hop to produce a beer of awesome full-bodied flavour with subtle cherry and rich fruit overtones."
So I've come up with this:
Batch Size (L): 21.0
Original Gravity (OG): 1.058
Final Gravity (FG): 1.015
Alcohol by Volume (ABV): 5.70 %
Bitterness (IBU): 31.6
Boil Time (Minutes): 60
3.000 kg Dry Malt Extract - Light (86.21%)
0.180 kg Roasted Barley (5.17%)
0.150 kg Crystal 240 EBC (Dark) (4.31%)
0.150 kg Crystal 150 EBC (4.31%)
40.0 g Fuggles Leaf (5.7% Alpha) @ 60 Minutes (Boil)
20.0 g Fuggles Leaf (5.7% Alpha) @ 15 Minutes (Boil)
Yeast S04 or White Labs WLP037 or Wyeast 1469
If you just want to get one crystal malt, use 250 - 300g of dark crystal. But the mix should work well. If getting both, don't worry about EBC, just get 'crystal' and 'dark crystal'.
The commercial description on ratebeer.com says, "Abv reduced from 5.7 to 5.6 in 2010. Its initial sweetness is, apparently, "of roasted and vinous notes with a subtle bitter aftertaste". Strong fruitiness, often with "banana notes" standing out, is derived from Old Peculier's fermentation process."
I've based the above on this info, and used regular crystal and dark crystal as I think this will get close to the OP colour and malt flavours.
Do you know how to do an extract brew? It's easy...
Boil about 10 litres if you can. If not, 6L is about the minimum, in a stockpot or similar.
How: Steep the grains in a grain bag at about 70C in this water for 30 mins, then remove and stir in 1kg malt extract. Bring to boil, add 40g Fuggles, 50 mins later add 20g Fuggles, 10 mins after that switch off. Stir in the rest of the extract. Cool in sink. Pour into FV via a sterilised sieve. Top up to 21 litres with cold water. Aerate with a sterilised whisk or similar. Pitch yeast around 20C.
This will be much better than a kit, and you will get a lot of satisfaction. You could improve it even more, and save a bit of money, by replacing 1kg of extract with 1.5kg Pale Malt (eg Maris Otter) and steeping this with the other grains but keep the temp between 65 and 70 for 45 mins by wrapping the pot with a towel or fleece.