Cam belts don't "slip", they are a toothed belt-the cam belt tensioner pulley bearings however can make a chirping noise, usually when cold. More likely to be a loose alternator drive belt, particulary if the noise starts immediately after the car is started from cold.

Get an independant workshop to diagnose the cause of the noise.

Just noticed that the water is below minimum could this noise be a water pump bearing or are they unrelated.

(sprayed wd40 on the external belt which i presume is the alternator belt (on left of engine) and the noise briefly went off then came back after 20 seconds)

Hmm, lubricant, like that won't have done it any good.

Sounds like it is indeed the auxialliary drive belt, once these start squealing its usually a case of replacing to shut them up, at the very least you will have to try and clean the WD40 off, that'll be fun getting it out of the grooves of the crank and alternator pulley.

The only stuff i've ever found to help shut squeaking belts up is french chalk.

Continental belts seem to run quietly, appear to be impregnated with French chalk.

I think I would double check the 150k or ten years.