I think the garden centres carry a specialist flowering cacti and succulent feed - make sure, if you find it, that it is for flowering ones though. I forgot to tell you, if you google the plant name Adenium obesum, it comes up with various things - select the Mr. Brownthumb one - its a forum about this plant and has useful info, even though he lives in the States.

I've been using an organic fertilizer called 'Raingrow'. Probably not available over there but it is a 4-3-2 rating. I think it was on Mr. Brownthumbs site that I read they like a higher nitrogen number and they like neutral or alkaline water.
I think Milky is using a cactus fert for her Adeniums and her seedlings grew enormously this year.