The transfer took the best part of 8 months with all of the red tape and he finally received the maps yesterday with my property marked on it, but he did say that Ulster bank never put a charge on the property and he has the deeds in the office. He said that both my old solicitor and Ulster bank are at fault as they never ensured the transfer was completed properly.
This is exceedingly odd behavour by your current solicitor, and he's now telling you it'sthe old solicitors and Ulster banks fault the mortgage is not registered. Nonsense. He surely knew not only was the property not registered but that you had a mortgage on it, it would be a very unusual young person who didn't have a mortgage, and presumable you had told him this, and he would have had the file I presume from the old solicitor, via the Law Society, or he would have discussed it with you.
I would think it's your current solicitors fault that he somehow forgot or omitted to register the mortgage at the same time he registered the property. He was after all hired to sort out the mess that your previous solicitor left you in. Excellent news that the solicitors insurance has paid for sorting out the error, if your current solicitor now wants to be paid 'extra' for the 'new' work of registering the mortgage, well I wouldn't like to be him dealing with the Law Society. He'll have some explaining to do. And he's going to be out of pocket I think as it's his own fault, and while he can do his own work, 'for free,' he'll have to pay the Land Registry fees which I think went up a lot in the last year or so.
In relation to your question about not paying the mortgage, why would you even ask this. You borrowed the money, you owe it, and if you don't pay it you can be sure they will go after you for it, and they will either go for an instalment order against you, or a judgement mortgage on the property etc. Just because they don't have a mortgage on your title/deeds doesn't mean you don't have to pay it. It's just currently unsecured debt.