One thing you've done wrong is prune too early - Hydrangeas shouldn't be touched before April, and late April at that in colder regions, so that any frost which occurs catches the dead tips and spent flowerheads from last year rather than the new growth beginning to appear. Even so, new growth on Hydrangeas is often nipped by frost - it usually goes crispy black rather than brown though, so are you sure you diluted the aluminium sulphate to the right strength? I have to also suggest that, as the plant is in a pot, I wouldn't have used aluminium sulphate, which is really designed for use on open soil - much easier to use ericaceous compost in a pot.

Bamboo, Thanks very much for your reply and advice. Would covering them with fleece etc help them at this stage or do you think damage is done and I should cross my fingers a hope it recovers? Also should I re-pot using ericaceous compost or will top dressing suffice? Regards

I'd repot completely, just in case the aluminium sulphate was too strong or something - covering when you know its going to be frosty overnight would help, but remember, even if it gets nipped by frost, it won't kill the plant.