i can recommend chefs vapour & alchemists cupboard,had a few different flavours of both & all were good quality & good service as well,i particularly like chefs blueberry c bomb has a sweet start then the citrus hits & its got a fizzy sensation as well!
If you want a good cheap custard try FPecigs Devon Custard. £8.50 for 30ml delivered. I've ordered quite a bit over the past few weeks and it's very good. They do other flavours too but I haven't tried them.