Worcester 350 boiler pressure low - warning light comes on so i top it up with water half an hour later the pressure drops again. Have checked rads and pipes no obvious leaks. Water seems to be coming out of overflow

Sounds like a faulty pcb,

Have you bled the rads of air

Check the pressure reading, should be about 1 bar when system is cold. Various models in the range of 24cdi. If you do not have a manual, go to,
http://www.centralheating.co.uk/Boiler-Manuals/worcester-bosch-boiler-manuals/worcester-bosch-discontinued-boiler-manuals.
Get the manual for your unit, to wards the end of install manual it has a fault finder, Also has a basic 1 in the user manual. Number of flashes how fast the light flashes which one flashing can all mean different, So get the manual and check, Some parts are easy to fix, as long as you do not touch the gas side the rest is plumbing. Come back if needed.

When water is heated it expands. To deal with this all boilers are fitted with a small expansion vessel which is like a reservoir which holds the excess water.

This expansion vessel is like a balloon in a metal pot. When the water goes in the metal pot the baloon squashes a bit to make room for the extra volume of water. When it cools the balloon pushes the water back into the heating system.

What has happened is your expansion vessel no longer works. When the water heats up there is nowhere for it to go so the pressure jumps up to 4 bar. The Worcester boiler lets high pressure water escape (most boiler allow this to go through the safety valve at say 3.5 bar).

When the water cools the pressure drops very low as the volume of water contracts.

The problem is your expansion vessel - normally a little red painted metal cylinder inside the boiler.

sludge in the rads sytem nees to be flushed out this can done by a company or drain down system and remove rads and flush out with hose

All the combis I've come across merely fail to fire if the pressure is not up. not only do they not fire, nothing else works including the pump and in any event the pump should not overheat just on low pressure.
water loss - try turning off the valves on the flow & return pipes [3/4 inch ones] 1 valve per pipe . This will isolate the rest of the system - rads etc Note pressure over a time period. In this mode the hot water circuit will still work but do NOT use the heating circuit
1/2 bar over 2 days is not serious but it should be found
Once you have either mended the leak in the rest of the system or found there is not one, insist on a replacement boiler
In any event having looked at the reasons given for the fuse blowing, I would definitely get a replacement.
These problems should not happen on a new boiler
I list Worcester.-Ariston-GlowWorm-Ideal- and many others who would confirm this

Pressure is ok? at 1 bar or above
I think on this one there should be a reset button [red] on the left hand top of the casing - or lower the control panel [ phillips screw on each side] and the button is on the left of the base plate. Press this and try the heating.or hot tap
P.S. if you're on a prepayment meter check you have gas.

check the tops of all your radiator valves for bypass if found, easy to repair best of luck

Manual here
http://www.worcester-bosch.co.uk/homeowner/literature/discontinued-boiler-literature/discontinued-gas-boiler-literature/24i-junior-rsf-discontinued-aug-2007-literature

All these machines need 0,8 bar minimum pressure for one.
When significant drop of pressure call qualified repair man.

If this is not your regular work you are at mortal risk.

http://www.worcesterbosch.com/Go there see if you can get some help.