You really needed the plastic strap that would have been looped around the cylinder between the reservoir connections for the full detail of what it was.
However - what kit car? Might be able to suggest the donor car and hence who to go to talk about replacements.
Cheers Bri, it's a Spartan, cortina (mk 3 based, we think).
I have put more photos on the kit car forum, should you like to look, i would have put them here aswell, but these things take me ages, and don't always work, lol.
Had the reservoir spring a leak on my kit car last year, just took the master cylinder complete to a local parts supplier, not Half rods, and at a glance he told me what it was from. In my case Sierra and IIRC replacement was around £80