I would use a medium coarse, fast-draining mix. Mixes that pack down and turn muddy are hard on citrus.

I have two citrus trees in ericasious compost. Would that count as medium course, fast draining compost.

Some are, and some aren't. Citrus does like a little acid in the soil, though. I have had luck with a mix of 1 part coarse sand, 1 part pumice, and 4 parts double sifted composted forest products--i.e., what will go through a 1/2 inch mesh screen, and won't go through a 1/16 inch mesh. I'm not sure what the names for equivalent products would be in the UK. Maybe one of the John Innes mixes comes close to the final product?

I was able to buy special citrus compost here - a very loose mix as citrus plants don't like packed soil. Maybe it is available in Britain if you Google it?