These boilers often leak inside on the left hand side.
Carefully check the 3 rubber hoses, the pump connections, the auto air vent on top of the pump, the reversing valve section below and the temperarture/pressure gauge connection.
If you notice any drips on the hose connections DO NOT be tempted to just "nip" up the jubilee clips without depressurising the heating side of the boiler first. Sometimes the plastic unions become so brittle they will shear off with the slightest interference so be prepared.
As for the intermittent hot water firstly check the operation of the diaphragm. Remove the two screws indicated (use 1/4 hex bits) and carefully take off the top cover complete with switches. Open the hot tap and you should see the levers inside operating. If they don't move properly and you can easily push the pin (at the back) towards the back of the boiler when the hot tap is open the diaphragm has failed and you will need an "arch" kit.
Not a particularly easy job (especially if the isolating valve jig is strained buy the pipework).
If the diaphragm unit appears OK then note the wiring and check the microswitches. Pull out the pins if necessary to check which tags are common, NC, NO etc. Check for less than an ohm on all relevant contact sets. Often they go a little high.
If you do drain it down check the expansion vessel, if you let the pressure rise to 3 Bar and the safety valve opens it's a difficult job to change it.
Just a warning...pumps are now about £145.00, reversing valves £60.00, set of hoses £30.00, arch kit £15.00. Apart from all the leaks they worked OK
This fine example was on BG contract, took then 6 visits to stop it leaking. All they have changed is the pump. If they had changed the parts listed above (1/2 days work) it would normally give several more years trouble free operation. Must be some very shortsighted management.
