How to sue a cowboy builder?

Hi All,
We had a large house extension built 2 years ago.
The whole job was a nightmare, with the builder pulling every trick possible to make it look like they were working on it, but in reality they worked at the slowest possible speed, with one or 2 men on the job, while putting people on jobs whose customers were screaming louder.
In the end, after about a year, it was done OK but only after a lot of supervision from myself, checking for bodges, etc.
Now, we have noticed that there is a lot of damp/mould at the base of a large window. The window itself is (I know) sitting in a lead tray so hopefully the problem is not caused by the usual "replacement window" bodge where a window has been installed badly and water ending up in the frame base runs out of the bottom corners. But somehow water is still leaking into the wall (plasterboard) at the two bottom corners of the window.
I have faxed them twice, and one of the faxes was copied by Royal Mail Special Delivery. I requested them to come and fix it otherwise I will get somebody else to do it and then recover the cost from them through the Courts.
2 weeks later I sent them a letter, stating much the same, again sent by RMSD.
What is the most cost effective way to go from here?
Shall I just get another firm to do it, or do I need to take another step? In the past, this kind of letter has always worked (and in fact I did have to use such letters to get the builder to finish off the job originally) and I have never had to sue anybody before.
They don't appear to be a limited company.
Thank you for any tips.
I think the fix will involve ripping off the plasterboard at the sides of the window, and seeing where the water is coming in. There is 50mm Cellotex in the cavity wall and they should have used proper modern methods of sealing off the end of a cavity. It's going to cost 100s minimum to get it sorted, probably into 4 digits.
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Employ (at your cost initially) a quallified building surveyor to check THE WHOLE job for faulty workmanship and items not meeting building regs then send a copy to the builder via your solicitor, charging the builder for the survey and the solicitors bill get the solicitor to put a charge on the builders house and property if the fault/s are not rectified to current building regs in 28 days that should givehim enough time as a decent builder could almost build a house in that time
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Why is this necessary?
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If it works it means there is a much better chance of recovering the money.
Peter Crosland
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Peter Crosland wrote:

This of course assumes that
a) the builder is not a limited company b) The builder owns his home c) that the home if owned by the builder has any aset value
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To get disreputable builders to sort out poor workmanship you need some leverage, many small builders ask for cash payments, and if this is the case with you then you have some, so go back to them and tell them what the problems are and that you want things sorting out quickly and properly. If they refuse just remind them how you paid and that you will be taking matters further if they don't. If everything was invoiced do the same, but if they refuse to rectify the problems tell them that you intend to contact Trading Standards for advice on how to further your complaint. Incidentally whether they gave you a written guarantee or not, I think TS will tell you that the builder will be liable for faulty workmanship.
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