Incorrect fees, charges and deductions by Barnard Marcus Surbiton.

I am using Barnard Marcus Surbiton as letting agent for a property (flat) in Surbiton UK. (I manage the property for my aged aunt under a LPA (Lasting Power of Attorney) as I live in Cape Town South Africa. I entered into a rental agreement commencing 30 Oct 2018. It started well as they did find a tenant quickly and at a suitable rent. But from there on their administration and accounting practices are abysmal. They produce numerous monthly statements which are confusing. They double charge - EG the inventory check was charged twice - once in Nov 2018 and again in February 2019. They charge for un-mandated services - IE they did an income and expenditure statement for which they charged me GBP 102 (This has subsequently been refunded. They take the full tenant finding fee up front which means they do not pay our any money for the first 4 months. They also took it upon themselves to deduct Income Tax and charge me a quarterly fee of GBP 120 for doing this. I have been trying to get this resolved for a couple of months now with very little success. They initially acknowledged that they were wrong to deduct tax, and charge the fee, but they make no effort to refund the money despite frequent requests. I have escalated the matter through their complaints procedure. They are now trying to duck and dive and get out of being responsible by saying they did not have a the LPA on record, which is impossible because without that I would not have been able to sign the agreement. They also say it is my responsibility to recover the incorrect tax deduction from HMRC myself. Very poor and unprofessional conduct and a disgrace to the industry. The solution is very simple - acknowledge your errors, create a statement reversing all incorrect charges and refund the money.