CH: hot water working but no heating

You've probably not had any replies as you haven't provided enough information.
You say the system is archaic so what does that mean? Is the hot water a gravity system with only the heating pumped?
Are both the hot water and the heating pumped? If both are pumped do you have a three port valve or two two port valves. What size shoes do you wear and what is your inside leg measurement? Over to you.
Edit: Never assume . . . .

 

Two tanks in the loft is a vented system. At 20 years old there's a good chance that it's a fully (ie hot water and heating) pumped system. So somewhere or other there will be some valves. Either zone valves x 2 or a three port valve. Have a look on the screwfix site to see what they look like. Zone valves are often near the boiler and the three port valve is often in the airing cupboard. Sometimes some moronic plumber has hidden them away (like under the chuffin floor in one instance). When you've found them report back. Once I know what you've got I'll be able to give you some pointers.

 

Those are zone valves. When the cyl stat or the room stat calls for heat it sends power to the motor in the relevant valve which opens the valve and once the valve is almost open operates a micro switch to power the pump and boiler. The fact that the hot water is working indicates that the pump and boiler are OK so that leaves the CH zone valve. Either it is not opening at all (stuck or failed motor) or the microswitch has failed.
Edit or the room stat is faulty and no power is reaching the valve.

 

Jason, yes, it should be fairly easy to test the wiring using a voltmeter - provided you don't kill yourself.

What you do need to watch out for is the strange wiring of your 'stat and the coloured wires which don't make much sense...

Looking at your 2nd photo in your first post, can you confirm that the terminals are marked;
Term 3 BLUE
Term 2 YELLOW
Term 5 EARTH
Term 1 RED

If so, then the best info I can find says that Term 1 (RED) is the LIVE supply to the 'stat (yippee - that makes sense )

Do bear in mind that this will (should) only become 'live' when your timer/programmer switches the CH 'on'. So, you can turn on your timer and test for 240V across Term 1 and 'earth'.

Then it goes all stupid. Term 2, afaIk, is the NEUTRAL to the 'stat - even tho' it's a yellow wire. You can confirm by testing for 240V across terms 1 and 2 - live and neutral. This Term 2 should always be 'neutral'.

That leaves Term 3 (blue wire) which is - I believe - your 'switched live'. This terminal should become 'live' whenever you turn your 'stat dial up until it goes 'click' and calls for heat. It should go 'dead' when you turn your 'stat back down.

Yes, I know it's coloured blue... It should really have a red sleeve (even tape) around it to show it can become 'live'.

I may possibly have got terms 2 and 3 the wrong way around, but I don't think so. (It is more 'usual' to make the neutral wire 'blue' and the switched live 'yellow', but even that is not perfect as all wires which could become 'live' should have red tape on them, and the neutral - with these old wiring colours - should have 'black' tape.)

If you turn on your timer to provide power to the room 'stat and first test for power across the Term 1 (RED) and 'earth' to confirm power, then move the 'earth' probe on to Terms 2 and 3 in turn - only one should also show 240V, and that is the 'neutral', which hopefully is Term 2 (YELLOW)

If that's all good, then move the probes to 'earth' and Term 3 and check that 240V appears every time you turn your 'stat up, and offski when down.

If that seems ok, then you've pretty much discounted the 'stat as being the problem.

Next step will be to check your motorised valve - see if it makes a noise (whirr) when you turn your 'stat up and down.

The motorised valves also have a manual control lever which you can use to 'force' it open and make the boiler come on - we'll come to that if needed.