Hi,
Firstly I'm sorry to say but it is extremely unlikely your truck is test exempt. The recent changes to the law seem unfair but they are quite clear as far as dates are concerned. The claim for test exemption when taxing the vehicle is a self-declaration and as such you can declare a vehicle to be test exempt even if it not, just as you could with the old V112G. The reason it is a self-declaration is so that the responsibility is placed solely on the person who declares it. If you choose to falsely declare it is at your risk.
The latest update to the HGV tester's manual indicates that your truck will be tested against the regs which were in place when it was built and the spec to which it was built. Therefore it should be perfectly capable of passing a test just as it could when new. There are a few items which have changed such as under-run protection and improved lighting/reflectors which are mandatory and must be retro-fitted, but all of these are detailed in the manual you have linked to and are not hard to comply with.
Regards - MG