When I popped round a few weeks later my dad said he had a lens for me that he'd got from a car boot sale.
A 50mm lens for my Minolta X-700. Doh!
Turns out they are different though.
The one I got off of ebay
The one my dad got from the carboot
The MD (above left) has a min aperture of f22 whilst the MD Rokkor (arove right) has a min aperture of f16.
Guess I'll have to do a comparison roll of film.