air con leak

[Bruno55]

I will be having the O rings on my air con pipes replaced next week as the system seems to have developed a leak.I have been told that if the O rings don't fix it,the pipes may need replacing.The price of these pipes is in the 300 Euro range (we now live in France but brought the Berlingo with us).I cannot see why these pipes should cost so much,is there somewhere back in the UK where I can buy them cheaper?
We will be returning back to the UK in a few months and can get our son-in-law to fit new pipes at the garage where he works.
If not the pipes,is there any other common problem to look for?
The car is a 2003 2.0 HDI.

[Coco]

There is a dye that can be added when the system is filled. This will show where any leaks are, used to be read by U.V. light.

Other than seals, the only problem I've had with air con failing was the compressor bearing - you'd hear that!! and a condenser went once, that meant me doing without air con due to the price of the part and dismantling the dash to fit it.

[Bruno55]

The mechanic looked all over it with the UV torch and the glasses but did not seem to be able to trace the leak.
I used to work on motorcycles for a living so know my way around engines so it did look to me like a bit of guess work.If I was the mechanic I would have charged the system up again which would have made the leak more obvious,but what do I know...................