Not unlike the roadcars, the balance between originality and performance / usability is difficult to achieve with the Gp.A cars.
From the collectability point of view, originality and attention to detail is key, and by originality I mean specification rather than actual physical metalwork (although there's only so far you can go down this path). If you want to go out and be competitive on track, then certain trade-offs must be made in this respect. In the case of the ex-Harvey Labatt's car, it's pretty much turn-key usable in fairly high-profile historic events. In order to be reliable and performant yet keep costs sensible, certain non-period modifications have been made to the engine, suspension etc.
This said, from a value perspective, originality still takes a back seat to the following criteria that I keep harping on about:
1. Who built it...?
2. Who drove it and in what series...?
3. How successful was it...?
You can have the most up-to-date, beautifully presented, top-spec car in mint condition but if it was built by Bill and Ben in their shed and entered once into the north-east Shropshire parochial country bumpkin street race to be driven by their mother and then after all that it failed to make the start then it's not going to be worth a fat lot in comparison with a car with real provenance and pedigree. In this case, the car in question has a good track record so no doubts to be cast there.
Again, I'm not saying the car isn't worth the reserve price. I don't think it is a million miles off at all. I am however not surprised that it didn't sell. IMHO, I reckon a few weeks with Paul to improve the presentation of the car and it'd sell.