Intermittent Air Con fault - Fabia SDI

The dashboard switch "AC" lights up when it is pushed (subject to the fan switch being in any postion other than "off").

With the engine running, this used to start the air con - the air coming through the dashboard vents would get colder

However,for the past few weeks the air coming through the dashboard vents has not been getting any colder

My usual (non main dealer) garage and an auto electrician have checked the car but they have not found a fault other than "no electrical feed to the compressor".

This morning, I pushed the "AC" button and the air through the vents did get colder!

I called into my usual garage on my way to work and they checked the pipes under the bonnet which were very cold and also used a digital thermometer inside the car. They said it was working properly

At lunchtime, I started the engine but it wasn't working again!

I would appreciate any advice offered.

thanks

Martyn