Sorry, but why would you want to hold back 10% of payment?
If you did not agree that at the signing of the contract then you probably can not legally do that now, the contract is what you both agree to.
Personally I think that is a bad idea, and it may well delay things for you, which time is not really on your side.
Building regulations, there is and has been so much confusion over this, and over the last 2 years I have gone into great detail with my local building control to try and clarify the situation, following my last conversation with them after they had received new guidance you do not need building regs, although it may be a good idea, they check for Part P (electrical certificate) Part C (water ingress) and structural loadings (which you should have calculations for.)
So what you should have from your installer is structural calculations (you need proof that your roof is strong enough) wind loading calculations, and a copy of their Part P certificate.
None of this will effect your FIT application, or will it effect how the system works, it may cause problems in the future if you decide to sell your property and a surveyor/solicitor asks for these documents.
Building regs in my area costs just over £100, it may be worth it for peace of mind, and they may pick up something that is not correct, however, they are not PV experts, just inspectors with limited training and ever changing guidance.
Just get on and have it installed, I am sure it will all be fine, you can post pics on here after, there are many that will find fault if there is any.
I hope this helps.