Hi Dave,
I can understand you want to swap the thirsty Hercules petrol engine for a diesel, and although it can be done I understand it is not a straight swap over.
If it is the fuel bills you are concerned about, you might want to consider fitting an LPG fuel kit. This conversion is quite popular in the Netherlands and typically lowers the fuel bill by 50%. I've read about specialists offering LPG conversion kits in the UK, so you might want to contact some of them to see if this could be the way to go.
Good luck,
Hanno from Holland