Hi Minnie123
Sorry to hear this. It's gonna be a bumpy ride I suspect. There's a few things from your post that I would pick up on.
1) Firstly you say that there were a list of complaints and you responded to each one with solutions. Are you happy that none of these grievances should result in non-payment? Do you feel that your offer of paying for materials only is fair considering the complaints? Are they likely to make a counter claim that outweighs the cost of materials? I just feel you need to be ready to answer these questions as things progress.
2) It seems to me that you should probably take advice on your Retention Of Title clause as I think that it needed to be very strong and clearly worded and make provisions for how the title of goods would be recovered later on specifically with regard to access to the property. I'm not sure about windows specifically but usually you can't leave a consumer is a worse position than before the contract so this is likely to mean you would have to put old windows back in. This leads to another consideration of whether it's really worth it to recover the goods. Apart from the feel good factor and possibly preserving your reputation, it might be more financially prudent to move on and make your money up on future jobs and chalk this one up to experience.
3) In any event, think about what you want to do in future if this situation arises with other customers. If you do want to effectively recover goods then I think you should get a properly drafted set of T&C's that specifically cover ROT.
4) Same goes for deposits, staged payments etc. I know you are probably kicking yourself for not taking staged payments but there are people out there who will take advantage of this over and over again

Have a very strict rule in place about credit limits and not doing work or parting with goods until staged payments are received.
5) Depending on which way you want to go now, I would suggest ...
a) Take proper legal advice on recovering goods - you can then refer to this in your correspondence prior to a MCOL. Either with Thomas Higgins or someone else. A personally worded letter that makes reference to your specific contract, sent from your solicitor outlining the sequence of events leading to their complaints, the solutions you have outlined and the legal steps you intend to take. If you are able to proceed with recovery of goods then these next steps can be outlined in the letter. This is likely to make your debtor sit up and take notice! Even if you are not going to pursue recovery of the goods on this occasion, I still believe a decent and specific legal letter is likely to illicit more response at this stage.
b) If you aren't going to pursue recovery of the goods and believe you are ready to simply issue a claim via MCOL then do this immediately when the 7 days is up. I believe in gut feelings and if you suspected that this customer was a non-payer from the start then you are probably right so be ready for their defence and any counter claim. It could get expensive over and above the cost of the initial debt claim. Issue it for the whole debt amount (not just the amount you were prepared to settle without prejudice) and include 'reasonable debt recovery fees' and interest.
c) If you are not sure about the MCOL just yet, another option would be to pick up a paper set of forms and fill them in, photocopy them, and send them with a covering letter as a '2nd Letter Before Action' that shows that you "really really mean it!". Sometimes this can have the effect of showing real intention to pursue your debt claim. (hopeful but you never know).
Good luck