I can't remember the exact details of yours but if it was after 6/78 don't bother as they don't have records past that time, plus anything close to 6/78 that is in the records (like May sometimes April) has bugger all data and many cars are missing as they hadn't been sold yet. The records remaining that they use are a microfiche copy of a z-fold printout service/warranty report so only vehicles actually recorded as sold and the paperwork made its way back to the zone office to be entered into the IBM machines are on the report. The missing vehicles will have been in the IBM but the report run only listed those sold. This is most likely why the odd XU1 or L34 isn't in the records as they were never "sold" and/or registered as such, went straight to race duties or otherwise somehow missed being recorded as sold.
All GMH need to look for your van is the VIN (all they actually need is the PSN), the rest is irrelevant to them. Give them the engine number to confirm too if it is recorded. All they can tell you is when it was finished, who sold it in what month and what mechanical options it had ie engine, box, front discs, air, steer and what rear axle was in it.