This happened to me all be it about 15 years ago when I had my identity stolen. I was advised to register with CIFAS and Equifax and dispute the fraudulent activity on my credit history. It took a while but eventually all the history was cleared.
Whatever the monthly mortgage payment is you then have to add -
Council Tax.
Energy costs; Gas/Electric/Oil etc.
Water Bill.
Buildings and Contents insurance.
Life insurance to cover mortgage.
TV licence.
Internet/phone.
Rainy day money for repairs and maintenance.
before you even think about...