Robert
I've had a very similar issue and think that I might have the answer for you...
F5 fault code ... if you've been adding rads to system or similar, then chances are you have introduced air into the boiler, which causes the thermistor to click off.
You need to take off the front panel (2 retaining screws underneath) and lift off. Then take off silver panel (remove retaining screws again). Top left hand side of innner workings of boiler is the overheat thermostat/reset switch. Two black plastic prongs sticking up with red button inbetween. Press down on red button - it should click. You have reset system.
Now restart boiler - turn central heating water temperature control dial (on LHS) to 0. Turn mains reset switch (RHS) to ON (1) position. Green light should come on and then go off. SYstem pressure should show on digital display. SHould read at least 0.7 bar.
Turn central heating dial (LHS) ON. The lighting sequence will begin, lamp will go green to show boiler has lit and you should be able to see the flame through the peep hole. Display should show system temp now rather than pressure.
Put the silver plate back on and then the front cover!
The manual can be found for 12/15/18/24/30/38hxi on the following web site: http://www.high-efficiency.info/images/hxi%20install.pdf
hope this helps
Jo Bourne
November 2007