Firstly have a look at this DVSA page. Scroll down to the bit about rebated fuel for agricultural vehicles. There is some relevance, despite yours not being agricultural.
I couldn't find any exemptions for repairs or relocating such vehicles.
My guess would be that your truck should have taxed diesel put in it to cover the on-road journey. Calculate the distance to be covered, and add that amount of fuel, from a source that can be checked, like a receipt or electronic system. Although the red dye will remain in the system, you would be able to prove that you'd paid the tax for that journey.
Better still, from the DVSA page, find the contact details and ask the DVSA. "I need to do this", not "I am doing or have done this". An email might be best, then you have proof to carry with you, but don't give the vehicle details until you have an answer you can live with. Don't want to make yourself a target.