The Stop lamp switch is activated by a bar welded to the brake pedal , it's very easy to remove and check but a little fiddly to replace, it's almost certain to be the guilty party in your case
.There are two types of switch fitted depending on whether or not you have cruise control , non cruise have one microswitch , cruise control models have two . The following is an extract from an article I wrote a while ago and should help you.
Push back the driver’s seat and adjust the steering wheel to maximum up and in. Disconnect the white multiplug from the stoplamp switch.
Open the bonnet, lift off the cover above the brake-servo.
Using an 8mm socket and some extension arms with universal joint, dismantle the two fixing bolts. Remove the old stoplamp switch from the cockpit.
Given the lack of space between the seat and the pedals, it is nearly impossible to see the spot where the stoplamp switch is fitted. I used a thin nylon string which I have passed through one of the screw holes in the bulkhead. I have attached one of the switch retaining threads then I pulled on the string to bring the switch into position.
Thanks to this I have easily manoeuvred the switch into position so the nuts can be put onto the threads and tightened.
Connect the multiplug for the stoplamp switch.
Hope this helps but PM me if you need further assistance/advice ,
Nik