ABS & ESP warning lights come on

[fully]

I have a 2000 Audi TT (225). Have had the ESP light and ABS lights coming on together. Thet come on most of the time but not all. Once they come on they remain on until the engine is switched off. I have had by dash replaced already but this has not resolved the issue. Audi believe it is one or both of the following sensors that need replacing:
a) steering angle sensor
b) longlitudinal acceleration sensor

Each will cost me £300 inc part and labour from Audi.
I have checked and all brake light bulbs are fine although have noticed that occasioanly the brake lights stop working!

Have seen a number of post about replacing the brake light switch so have decided to give this a go first (to save the £600).
Brought the switch today but was concerned when the chap at the Audi parts counter stated that you only get once chance to fit this sensor? Something about getting the 'setting' right and if you don't get it right then the switch is useless and I will have to buy another? Is anyone else aware of this 'setting'? Anything I should know before attempting to fit?

Thanks