Probably £4,000 or £5,000 max in mint condition, but it was obvious it wouldn't go cheap. Can't be many 1973 ones left now. Nice find though. There will definitely be a bum fight it this re-appears on ebay!
It probably has also dropped off the DVLA register and PNC as it has been garaged for that long, which shows that DVLA records and how many eft arn't that accurate anyway.
Tom