F 13 error on a glow worm 38cxi?

My 38cxi is approx 3 years old and I have had the normal problems with the boiler. F14 which is an over temperature error, which generally means removing the heat exchanger and washing out the old lime scale and debris that has been in the system over the last 15 years. Which I cured by fitting a hydro flow coil near the boiler on the CH return, which I must say really works and worth the money I paid for it!
I was always stripping the boiler down practically every 8 weeks, now its only 1 once a year.

I now have a fault code of F13, which in the paper work that comes with the boiler. Indicates that it's a connection problem on the main PCB. I have reseated the all of the connections on the board several times and checked all the leads which all seem ok.
I have isolated the fault to the DH circuit, as if I turn off the hot water dial to zero and wait a few seconds the error is reset and if I return the dial to the required hot water setting I will get hot water again.
The error is intermittent and starting to get to me,
I suspect the main PCB, but over £125 I am reluctant to go and buy it, if it is not going to fix the problem.
Any Ideas or advice?