I'm nearish you in Sudbury. I have found Cats Rats & Elephants on A12 near ipswich are very helpful as are Kesgrave Aquatics which is otherside of Ipswich. Don't think either have convicts in, but both have quite a decent stock of fish, although mostly common types.
What decent LFS are there in colchester? I've been to seapets but the experience was a bit meh (Although bought my Roma 240 from them which was a good deal!)