OK, there is a lot of detail in the complaint you are correct!
You need to send them a formal letter before action, setting out the details of the claim, the amount you are claiming for and why. This is different to a complaint, and you need to be clear as to what you are expecting from them in order to resolve it.
You give them 14 days in which to respond before you take the matter further and consider issuing a court claim.
I think that you should do this via a solicitor though, rather than in person. It should be possible to settle this through negotiation and avoid going to court altogether. A strongly worded solicitor's pre-action letter should do the trick.
A court application will cost quite a lot, given that you pay an application fee of a percentage of the claim value.
So you would be better off spending a little money to instruct a solicitor to write to them at this stage.
Can I clarify anything further?