Hi mk2 and Rich,
Thanks for your reply - I thought I'd post this as a follow-up to my problem.
Dad I got round to trying a clutch cable earlier (last owner bought it but never fitted it) - it seems much better now after a good 30 minute drive. What we found is that the end of the sleeve on the cable by the ball joint which connects to the clutch pedal was all warped and damaged, suggesting it wasn't installed properly the last time someone did it (if ever!). Also, when the cable was out of the car, I tried pulling the cable itself. It pulled up fine but did get stuck going down suggesting it was frayed inside
New cable seems to be doing wonders now