[Laird Scooby]

Get a lewngth of fishing/strimmer line Chris, loop it round the wires between body and tailgate and shut the tailgate witth you lad inside, holding t'other end of plastic line.
Now you go and jump on the brake pedal and get him to try wiggling the wires by using the loop of line as a "remote" finger. Maybe you'll need to try this in different places to pull the wires in the right direction, possibly even with him laying on the roof (spread-eagled to spread his weight as evenly as possible) but once you find where and what direction causes the 3rd brake light to work again you can then narrow it down to which wire it is.

Alternatively unclip the 3rd brake light and tuck the wiring into the tailgate for the MoT - it's not an obligatory lamp and if it's not fitted, it can't be tested and therfore can't be failed. I have a feeling if it's not working at all that still isn't a fail but cars with multiple bulb 3rd brake lights (eg 7 bulbs in a row) have to have more than half of the bulbs working or none at all to pass.

Note that some time after your car was made i believe the 3rd brake light became obligatory so different rules may apply to different cars.