Sorry to dig up an old thread, but I just finished a keg of an excellent Fuller's ESB clone, so I thought I would contribute.
In addition to the pale malt and crystal malt, I'm pretty sure I taste a little bit of flaked maize sweetness, and it is mentioned in some interviews online with the brewers at Fuller's.
5% 75L crystal sounds a little light on both color and flavor to me; I would go for a larger quantity of lighter crystal malt. This is what I use, and it is pretty spot-on:
Grain Bill:
9 lbs. Maris Otter (I use Warminster's, but anything at ~4.5L should work)
1 lb. Flaked Maize
1 lb. 55L Crystal Malt (I use Simpson's medium)
Hops:
1 oz. EKG (5.5%AA) @ 60 mins
1 oz. EKG (5.5%AA) @ 15 mins
2 oz. EKG (5.5%AA) @ 5 mins
Yeast:
Wyeast 1968 London ESB
I do a single infusion mash @ 153 for an hour, but with the flaked maize in there it will probably clear faster if you do a protein rest first. This should get you to about 5.5% ABV, 40 IBUs, and 12 SRM. (A little dark, I know, but the amber color looks really nice.)
One thing I will warn you about is that 1968 can get pretty active at warmer temperatures and will attenuate up to 75% despite the stats. Because of this, my last clone ended up a little boozy, closer to the bottled version than on tap. I probably hit about 72 at some point; 68 should keep it within the desired range.
-Shawn
