Mo condolences for you loss Miffy.
I don't know much about the 300 series cars, but this information may assist others in helping you with a value for the motor car:
https://cartaxcheck.co.uk/free-car-check/?vrm=UJR611Y
It appears from the records to be a genuine motor car in very good condition. It is a 1983 registration, and so will qualify as an historic vehicle (tax and MoT exempt) on 1 Apr 24. It has had the same owner since 2011 and sonce then the annual mileage has been very low (it is only 61,000 miles now). The MoT record is very good indeed.
If this was a 200 series motor car I'd say it would be worth £5,000 - maybe even £6,000. I'm not sure where 300 series motor car prices are in comparison.
Good fortune with the sale.
Alan