Happydonkey.co.uk will sell you all the stuff you need, including puly caff and a blind basket for backflushing. All that backflushing sounds rubbish to me though.
But I just take the opportunity of taking off the shower plate and the shower block (just undo the 2 allen bolts and prise it off) and de-gunking it all manually, and running ordinary descaler through.
Last time I left it so long that bits of scale dislodged and got stuck in the works, but poking about with a cocktail stick sorted it.
I just bought a new rubber seal, shower screen and cleaning brush from happy donkey – well worth it.