There are very few allowances left to claim these days and you can only go back 4 years, it's quite easy to do yourself, you could call in or ring your local revenue office for a bit of advice.
As for a mortgage there is nothing an accountant can add really, as paye you are going to be asked for p60s & payslips and a form filled by the employer together with bank statements of savings and loans, you are going to have all these or will have to get them, routing this through a third party that is not actually involved in the mortgage process unlike a broker is pointless and paying money for nothing in my opinion.