Andy,
The only true serial number on MG5 would be on the plates. The plates were hand stamped after being fitted to the chassis and in some cases it is possible with careful cleaning to find a faint impression in the chassis. This can be enhanced with chalk or a digital camera changing the contrast results.
The only other approximate date can be located on the edge of the crown wheels in the form of an etched or forged number in the form of 6-50 which is June 1950 do not confuse with 34/9 which is the number of teeth on crownwheel and pinion.
This is the date of crown wheel manufacture but would be within months of tractor build.
If you need more detail contact me on njarrett@btinternet.com as I hold the stock of new spares.
Neil