Consider employing a CIPHE professional to give you an unbiased written opinion as to the
real cause of your problem:
http://www.ciphe.org.uk/find-a-plumber/about-find-a-plumber/
Speak to your local CAB:
http://www.citizensadvice.org.uk/
Contact Radio 4's 'You and Yours' consumer programme:
http://www.bbc.co.uk/radio4/features/contact-forms/you-and-yours/
(Select the 'Tip Off' category. Keep your report short, just making it clear that you're getting stuck between the manufacturer, who blames the plumb, and the plumber, who blames the manufacturer, with the problem being compounded by an engineer who tells you one thing but who then tells your credit card company something different. Hopefully they'll then contact you to get the full story).
J T Spas trade from Leeds (although their registered office is in Northern Ireland) so contact West Yorkshire Trading Standards:
http://www.ts.wyjs.org.uk/wyjs-trading-contact.asp
Keep hassling!