What I mean is, Ian, that the sort of modifications you made to your car, were 'in period'. They were not provided by the manufacturer but were available from somewhere like Halfords during the currency of the model, or, during what was considered to be it's reasonable or natural life expectancy. Thus, in the 30s one may have bought a gear lever extension for an Austin Seven, may have fitted 'helper springs' to the rear axle, purchased a bulb horn or even fitted an overhead valve cylinder head. In my view it is legitimate to present the car with those kind of enhancements – as you are doing – but, again in my view, it is not legitimate to be fitting to an Austin Seven pistons from some 1990s Japanese model of car or lawnmower, giving it a clutch from something similar Oriental or introducing carbon fibre components to its suspension. All these practices are relatively common, nothing to do with 'in period' modifications and hardly in the spirit of the hobby.
What I meant by 'this sort of thing' is, as I said in my original post – someone looking at my project car will immediately notice that the fan pedestal did not originate from either Birmal or the Austin Motor Company. They will suspect that some 'Joe' made it in his garden shed and, in the vast majority of cases, will not be impressed.On the other hand, I could have bought a new pedestal that would be indistinguishable from Longbridge's and look, although not be, original. And, as you say, the 'value' of the vehicle is influenced 'by this sort of thing' – by my pedestal and all the other little bits and pieces I have made – oil filler caps, petrol filler lids, control pedals, oilers etc etc. But then I don't want to sell the car!
Hope that helps clarify.
Martyn