Not SE1 based, but in Battersea, is someone I know and have used before called Sarah Nops. She used to work at Good Housekeeping as a cookery consultant and tester and she made an fantastic weekend's worth of food for 12 of us having a country house weekend. The costs were very reasonable, especially when compared with Waitrose entertaining ranges etcetc, and the quality.
Her email is [email protected] and she's very nice too!
Tell her I sent you...
I can recommend Karen Liddell-Grainger on 020 7565 0185 or 07860 400569. She recently did a friend's birthday party at Greenwich boating club and everyone raved. I have known her and family since early '90s.