XJS 3.6 Coupe brake light failure

[Wilvene]

My 1989 XJS went in for an MOT and failed due to the brake lights not working? The usual checks (bulbs / fuses etc) were made. The garage (not a Jag specialist) confirmed 12v was going to the switch but not activating the lights? The switch appears to be working so the problem possibly lies elsewhere?

The car has been on SORN for the winter and was fine before being taken out with everything working?

Any suggestions?

Tony Brown