Your local council will almost certainly come out and test it for you for free.
Look up environmental health on your local council website or in the phonebook.

Thanks fot that but........
My local enviromental health officer is a bit too friendly with the local Welsh Water guy and i also know that all samples go to the Welsh water lab anyway so it would not be in their interest to find anything wrong.
I need somewhere to send my own sample.
Thanks

I would assume though that it is not Welsh Water that is polluting it and it would be in their interest to find any pollution and sort it out.? The other course of action is to contact the Environment Agency (if they exist in Wales that is) and get them to test it.

The EA should respond to any alert to waste flowing into watercourses. They should do sampling and are in charge of sorting out the person who is polluting the waters.

The problem you have is that you can only test FRESH samples of effluent or contaminated water. Samples more than a few hours old or which have been kept at the wrong temperature are void. You would not be able to send them by post, but would have to deliver them yourself to the lab.
We have our sewage treatment plant and septic tank samples tested locally to where they were collected.
For Wales, there are five NPHS centres http://www.wales.nhs.uk/sites3/page.cfm?orgid=457&pid=30036#ban
providing microbiological services linked to testing food, water or environmental samples. Four centres (Bangor, Cardiff, Carmarthen, Rhyl) collectively provide a wide test repertoire working to common methods and can give advice on sample collection, appropriate tests to undertake in a given situation and any relevant statutory tests.
 
For details on wastewater legislation, please see our website http://www.wte-ltd.co.uk or Tel. 01757 288022
 
I hope this helps.