Boilermate 2000 heating constantly on.. update

Hi i have just moved into a "new" house with a boilermate 2000, the heating is constantly on, all 4 LEDs are constantly on on the PCB.
The mechanical clock looks like it has been replaced at some point and is wired up to 1,2,3 and 5 and the neon is not lit.
Could this be causing the problem? If so can i just wire the clock up to 1,2,3 and 4 with the 1 and 3 connected
Any help would be greatly appreciated.

Please see below for picture of wiring

An update on what's happening to the boilermate...
I replaced the timer with a second hand gledhill xb215 from eBay, had no effect...
Replaced the acb with a new one..
The heating now can be turned on and off at the room thermostat and the clock, and only a max of 3 LEDs ever come on...however
When the heating is turned off we have no hot water, at one point a her error was shown when pressing swh 2 but is not constantly there, I will be replacing the dhw sensor in a day or so... Is there anything else I can try?
In addition I looked on the f & e tank and it appears to be filled with a film of sludge see pic... Is it safe to remove this?

Again any help or information would be greatly appreciated.
Thanks
Dan