One of the mistakes I made was not to inform Birmingham Trading Standards as soon as I had to start visiting Mr Steven P Sparrow of Go LPG more than 3 times. The main reason for this was that he kept assuring me that the next time he would do something else to get it working right - this, of course, never happened - it never worked right.
I did, however, several months after the conversion, get in touch with them and a case was lodged with the then Acting Head of Trading Standards and Licensing in Birmingham, Chris Neville.. The highlight of this letter, which did not help me, but could be of help to you, is to complain to Trading Standards within 3 months as otherwise they cannot prosecute under the UK Road Traffic Act 1988.
Incidentally, back in 2003, Birmingham City Council recommended that due to the sheer number of complaints about poor LPG conversions:
The City Council is not happy with the conversions and they have had a series of complaints.
Their recommendations to overcome this problem are as follows:
Interestingly, Autogas Worldwide Ltd (AGWW) [the suppliers of Romano LPG conversion kits] have stated, in the same paper that: 'LPG boost programme providing little help for converters therefore having effect on safety.'
Source: Department for Transport (UK)