leblanc DVM 4.20 in Surbiton - DHW but no CH

Installed c1988, was working OK through March 2006.
DHW still works OK, but at recent gas safety check the
engineer noticed that it does not respond to Central Heating
demand. However, he is unfamiliar with Leblanc kit.
Have user and installation manuals, but not obvious.
Pump does work (e.g. at start-up); main burner fires up
in response to hot water demand but not for CH.
(CH temp is high, and external timer is 'on'.) No room stat.
Anybody got any thoughts on this?
Or anybody nearby feel like taking a look?
Any help or ideas would be appreciated. Many thanks.
(Can tell-me-phone london ate tree nein nein too oh sicks tree)

Problem with these is that there's not a lot of 'em about so few of us on here could be specific without looking at it.
A starting point though is to check that the power comes out of the programmer when you turn it on.

Thanks for your contribution, Slugbaby, it's always worth checking the simpler possibilities first as per your suggestion.
I have difficulty "reading" the diagrams, but will try to plough through the manual.
Meanwhile, the pump runs (or appears to run), provided the control is set to "water+heating" AND the external timer/programmer is "on", but this sound stops if the timer goes to "off".
That seems to indicate "power comes out of the programmer", but I'll put a multimeter on it tomorrow, plus carry out any other checks that might get suggested.
So maybe there's a flow (pressure differential?) detector or something ...?
There WERE ten of these boilers installed in the block (of 10 flats), but mine is the only survivor. Almost reminds me of that song
"With one man of her crew alive,
That set to sail with seventy-five."

There is a lever on the pressure switch on the bottom right hand side which is almost certainly sticking and needs a bit of a waggle. Give the microswitch a couple of presses aswell.

These are a rather quaint design made up of unique bits never seen elsewhere. Spares are difficult/impossible to obtain.

Chris R operates in your county but I dont know if he would "come and have a look" as you so casually put it.

We would always charge for any diagnostic visit !

As yours is the sole surviving Leblanc in the block then that should give you a possible indicator that this is the time to replace ???

Tony

Oilski, you're a brick! Thanks VERY much.
It was indeed this pressure_switch_lever_microswitch you speak of,
so the problem is now SORTED.
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Thanks also for your comments, Agile/Tony.
I DO KNOW that a charge would quite rightly be made for a diagnostic call-out (unless absorbed into the cost of actually carrying out a significant or major repair), but such a call-out charge could be money well spent.
And I am very much aware that this GVM 4.20 is on borrowed time - "18 not out", or maybe even "n-n-nineteen not out"!
I have heard of 2 or 3 places that purport to hold a decent range of spares for it, including uk-plumbing.com and hrpc.co.uk, but the time will come when this boiler needs something expensive or non-available.
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Anyway, folks, thanks for your time.