Not really my area of law either, but the case for the police and the case for the bank are two seperate things. You cannot control whether the police take this forward and prosecute XP, so you need to convince the bank to back off first. This is your main problem - the bank would send in the bailiffs, it would be them ruining your credit rating, so that needs to be sorted out with them. Ignore the police for now.
Get advice from the CAB too and maybe see if you can get legal aid. If it is in a town with a university, you will probably find a law centre which will be able to help you. Suggestions below are about getting the bank to take notice, although I don't know how legally effective it will be - CAB/your solicitor would advise. Half the battle is being confident enough with the banks and taking control and getting them to prove that it's you (which, presumably, they won't be able to do!).
#1 get your credit file from Experian (I think you've done that) and check it. Alert them via your solicitor in an official letter that there is believed to be fraud and point out mistakes on the file one by one - always ask for a response.
#2 build a case yourself. Your solicitor will do it too, but high street solicitors are often very busy and have hundreds of clients and balls get dropped, and you want to make sure you have all the ammunition you need.
#3 Get "certified copies" of your passport signature page (handily on the same page as your photo) and copy of the specimen signatures from your regular bank. You need to prove what are your official genuine signatures. NatWest certainly has a specimen signature on their computer system that they check when you withdraw money, although the bank may be difficult about giving it to you. Your solicitor should be able to certify the photocopies with a stamp. The certified copies look more impressive than photocopies and can be shown to the bank, perhaps together with an official looking statement saying this is not your signature, you know nothing about the loan etc etc. Again, not sure how effective it is but you will look like someone who means business.
#4 keep photocopies of EVERYthing you send/receive (including letters addressed to your XP as evidence!) to prove your case - solicitors letters, letters to the bank, letters to your XP etc etc. Keep a note of the date and time of all calls with the bank and the credit reference agency and keep that with your file of papers.
#4 if the bank try and enforce proceedings for £1000 or whatever, contest. One point - is this bank chasing you also your regular bank? They should be able to provide evidence as to how they checked your identity when the loan was taken out in your name. Banks are bound by money laundering regulations whereby they should have checked your identity with photographic proof, although I am hazy as to whether it will be the same case if you are an existing customer and they may have the details. If you are an existing customer, they should have a specimen signature on the file for you anyway (see above).
You should be able to get your name of the joint account if XP signs a letter confirming it is wholly his responsibility -sounds unlikely that he will this willingly though. Your solicitor will draft it for you. I have literally just been through this (as in 1 month ago!) with my parents because my feckless father did exactly the same thing to fund his champagne lifestyle and misstresses leaving my mother in the sh!t. I am not proud of this, but basically blackmailed him into signing the letter by threatening to tell his new girlfriend about lots of his dark little secrets, so he did sign it, the bank accepted it and he has full responsibility for it now.
Good luck with everything and let us know how you go.