I have had my Worcester heatslave for 35 years and have had the same problem about three times in that period each time all that was needed was for the thermal coupler to be replaced you can get this from plumb centre {£10-00} whilst it's not the original part it is compatible
this is just the copper pipe with a sensor at one end which hangs over the pilot flame and when it is hot enough it builds up pressure in the pipe and opens the gas valve it also comes with the nut and olive at the other end nothing to hard to replace as it is out with the old and in with the new
professional would do this in half an hour did mine myself but new regulations mean that you need to have a registered corgi gas fitter to carry out the work and issue a certificate for the work carried out
or your house insurance could be invalid in the event things went wrong
rodders
July 2015