Correct, there is inheritance tax on lifetime gifts on a sliding scale but only on gifts made within 7 years of death. Anything given away before that drops out so you can forget it. It's just to stop you getting away with avoiding IHT by giving everything away on your deathbed!
So how much did he give away within 7 years before death, add that to his estate, and if that's still under the threshold you have nothing to worry about. Bear in mind also that anything he left to you totally escapes IHT because you were married (this is the spouse exemption), so when you're working out the value of the estate, you can ignore that completely for calculating the tax. THEN you deduct the threshold as well and see if there is anything still left. If most of the estate goes to you, or if he didn't leave a will it all automatically does on an estate that size, I feel reasonably sure there will be nothing to pay.
If there is still an amount left and there actually is some IHT to be paid, then we start having to consider who owned what proportion of your joint account. It's not automatically half. If, say, he worked and you didn't, so he basically provided all the money in the joint account, it's all taxable as his. If it's more complex than that, HMRC will not bother with the exercise of trying to calculate who put what in down to the last penny, and assume that you owned it half each. So any gifts made out of it are half his. If you paid for the boiler out of your own account, that's definitely a gift from you and doesn't count. And yes, then it comes within YOUR 7 year time frame (and hopefully, with all best wishes to you for your good health, will also drop out in the course of time). If you put the money into the joint account first, maybe for ease of payment, it's still your gift and doesn't count.
There may be no IHT to pay but you'll still have to consider what proportion of the joint account was his, and work out the value of lifetime gifts, in order to fill in the IHT return. It's all still got to go down so that HMRC can see and come to the same conclusion. If there's definitely no tax to pay, it really isn't going to matter much if you or the solicitor get it slightly wrong, is it?
I AM an accountant, and it would be nice to be paid for this, but you can have it as a freebie :)