Hi,
Thanks for your input. This is the instructions with a circuit diagram
http://www.screwfix.com/sfd/i/cat/pdfs/36/p4734536.pdf
I'm no expert so confused with whether 'N' is actually at term2 or term3.
Same thing happended to me last week - replacing the same Potterton PRT100 with the Honeywell and blown the 3A fuse on the timer/program switch.
I replaced the 3A fuse and reconnected the old thermostat to make sure the boiler is still working ~ phew!
Not tried any other combinations yet...