if the v5c isnt correct, then i'd write to the DVLA and inform them it should be petrol, not a diesel. not that it should make much difference, the road tax for a 1998 car will be based only on engine size anyway.
I've looked up the reg with the 2 vehicle check apps I have on my phone. Bizarrely, one says it's petrol (Check Car Details app), the other says it's diesel (Vehicle Check app)! Clearly some wires have been crossed somewhere.
A couple of years back I noticed that my Range Rover correctly showed as petrol on the DVLA ‘MOT history’ lookup, but as diesel on when you just went to view the ‘MOT status’.
Sent them an email and they corrected it in a couple of weeks.