Oddly enough, the classic car dealer in Diss who posted this on his facebook page as being 'available from him soon' advised me he would be asking £23.5k in as is condition due to the volume of interest he's had in it. He'd be looking for more if he recommissioned it.
I think that's a little optimistic, especially bearing in mind the Carnival red Special that's been in a dealers in Scotland for the last couple of months for £21950 (albeit with some paint defects that would require respray). As you observe there's a few TC's with large price tags on ebay/Car & Classic etc that have been unsold for some time, as well as some Specials with even larger numbers next to them, although the premium usually carried by specials is fairly good.
When it comes to auction results it's a little less conclusive. A running and complete car with a retrim and fairly basic respray but slightly chequered history failed to reach reserve in September at ACA in kings lynn at £10750 (reserve was about 14 as far as I could ascertain) and the sales from June to last month have been demonstrating a fall of 10-15% with the cost of living crisis impacting hammer prices. I think Mastersons had a couple of TC's and a S2 last year, with Prices under the 20k mark for them. Similarly a tidy but well used Pistachio green car at a dealer in Hampshire was at £19k from March to June last year before selling (possibly following some negotiation).
Your fathers car is clearly an excellent example, with a history that will definitely increase the value. In recommissioned state I'd estimate that it would comfortably exceed £20k, probably closer to £25k as it would be a usable car in grade 1 condition but usable as such as opposed to a concours show car.
Bearing in mind that as the car has been stood for some time the Achilles heel of the twincam - the water pump - would likely be a necessity for preventative maintenance, but that requires the removal of the engine as it is tight against the cabin bulkhead. This would factor as a consideration of the recommissioning process as it's a faff of a job. (when I had to have my last TC's done in 2010 it was £100 in parts and £1200 in labour unfortunately).
I did enquire as to the car with the dealer in Diss, as I am looking for a running G2/G2+ condition car as a usable classic currently but the condition and history of this car exceeds my current budget (especially if said dealer is expecting the price mentioned above).