I actually own number 29 Brooklands and from what you describe, the very same thing happened to me last year.
Clutch pedal wasn't disengaging and like you say, when attempting to change gear didn't firm up straightaway more like after 30%
I was fortunate enough to make it home. Ended up being the master cylinder which needed replacing at a cost of around £200 supplied from M3W Services in Swindon UK and relatively easy to fit
Like someone said tho if slave cylinder its an engine out job. I recently had the Bleazey Centa upgrade and found one of clutch plate springs had snapped which is quite common. Phil Bleazey or M3W Services can supply upgraded clutch plate.
Good luck
