Apparently so Mike, you now have to provide a receipt for the engine if you change fuel types, but I've just found this which says Historic vehicles are given an easier time, maybe that's why yours wasn't questioned.
The February newsletter of the FBHVC announced the DVLA change of policy for Historic Vehicle engine changes and anticipated that there may be difficulties:
"Verification of Engine changes.
This was a subject that has been causing some difficulties. DVLA have now made a statement as follows:
‘I note that the issue of evidence to support notifications of engine change also features [in the list of questions]. As you are aware, this was raised at the recent meeting with the Minister. I am pleased to confirm that this policy has been reviewed and that it will no longer be necessary for keepers of vehicles in the Historic class - date of manufacture before 1 Jan 1973 - to provide evidence to support an engine notification. Grateful if you can pass this information on to your members’.
All credit must go to David Hurley and Geoff Smith of the Federation for being able to do promote a ‘top down’ approach to resolve this problem.It is anticipated that DVLA will need time to brief their staff of this change in policy, and update their internal and external documentation.
In the unlikely event that an engine change for a vehicle with the taxation class of Historic Vehicle is rejected by DVLA, please send a copy of the rejection letter to the Federation. This should enable the Federation to have the rejection letter retracted, and enable DVLA to re-brief the staff member who sent out the rejection letter. If it is a ‘verbal’ rejection, then the name of the DVLA staff member and the name of the office will be helpful."
The Federation Secretary is Rosy Pugh, Stonewold, Berrick Salome, Wallingford, Oxfordshire. OX10 6JR
EDIT: that quote was dated July 2011so I guess they haven't got around to briefing staff yet!