Hi Tlo, what stage is the build at? Has the body already been bonded onto the Metro donor or do you even have a donor with the body?
As Gary J points out above, Banham's kits were a fairly straightforward build concept - take a cheap and easily available donor car, unbolt/cut off wings, doors, roof, windscreen etc., to leave a driving 'moon rover' tub, then bond/bolt the replacement panels in place, paint, trim and enjoy driving something completely different. The finished car just needed a conventional MOT and the detail changes amending on the V5.
Again, as Gary rightly points out, the kits were more about looks than ease of maintenance and if you do continue building your Uncle's X99 you should allow for access to service components, brakes, fuel tank etc.
The biggest headache with building any Banham kit today is IVA, which, if you're not already aware, is a 'super' MOT required where a car has had its chassis or unitary body radically altered during the rebody process.
https://www.gov.uk/vehicle-approval/...hicle-approval
As I
think the Metro donor is a unitary body with subframes carrying the engine and suspension, IVA will be required and it's an expensive and frustrating process building the car to comply with the rules.
Having said that, it's not impossible to get a home built special through IVA, but it helps if the car was designed to comply from day one, which Banham's cars weren't (IVA was only introduced in 2009 and Banham models were all out of production before then.
As an alternative, you could find a donor with a separate chassis and similar wheelbase to the Metro (89.3"), add a tube/box section frame, bond on the body and use the DVLA rebody rules to register it with the donor's identity but with a new name.
https://www.gov.uk/vehicle-registrat...tered-vehicles
One final thought, I fully understand if your Uncle's project holds some sentimental value for you and you want to put it on the road as a tribute to his memory (assuming he's passed), but the X99 was effectively a copy of the original Audi TT - indeed VAG stepped in and Banham had to cease production (see ongoing discussion regarding the DNA 250 California).
It might ultimately be simpler and cheaper to buy an early TT coupe and spend the time and money looking after that instead.
Anyway, good luck with the project and there's always advice to be had from some very skilled car builders on the forum.