From the tee piece the pipe should go to a actuator it looksa bit like a solinoid valve it should have 2 pipe conections and an electrical connection or in your case 1 pipe connection its between the vacum pump andd the brake booster

Cheers - I will have another crawl underneath tomorrow

I know this is an old thread, but thanks for the info as mine needs replacing now as the solenoid just makes it spin rather than engage a lot of the time. I've got a new unit (about £75) I just need the enthusiasm to try and get it fitted now, as I've also got the two front bags to change today....deep joy!