Baxi 105HE Heating fault?

I have a Baxi 105HE combi boiler.
When the hot water is in demand the boiler will run ok no matter how many times it is required
However when the selector is placed to the heating position, the boiler will heat up to the required temp but when it goes to re-heat it ends up locking out and the fault light on the 30degree light. On trying the reset switch, it relights but after 2 or 3 attempts locks out and the boiler will not reset until the temp drops back to around 40 degrees. I have all ready changed the condensate drain i and this drains ok.
Would the fault be on either the central heating temperature sensor or the safety thermostat....I hope so as thier cheaper to replace than the PCB!
Many thanks in anticipation of your help and to stop the wife hitting me when I swear at the thing

Paul Keenan
October 2008