I knew doing a joint one would add complexity and trip me up. Should have just done it all in the Mrs Name. It also says:
Which sounds fun, dealing with HMRC. Basically prove that the Mrs put all the money into the account, which came from her current account, which will come from one of my accounts anyway. So yeah I can "prove" she made the final deposit into the account but meh....faff.
I think we will just use the joint one for quite literally every day saver duties and then take out a higher rate fixed one in her name for a longer term store. In any case, we're only just going to be coming close to worrying about paying any tax on interest gains anyway, if at all.