Thank you for your expertise so far!! Great to wake up and see the replies and feel like we are getting somewhere.

Ive looked closely at the steerer tube and there does not appear to be any markings on it.

Here are some dimensions I've taken to help out. I don't have the best instruments for this so they may be +/- mm!!

Top tube 25mm

Downtube 28mm

Seat tube 29mm

Internal seat post 27.2mm

BB width 67mm, 38mm external, 34 mm internal threads, British thread.