Advice please- I have a very old central heating system with no thermostat. Replace?

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I have recently bought a house with a 20 year old gch system. It is the type with a boiler and separate hot water tank. The only controls are a mechanical timer clock on the hot water tank and thermostats on the radiators. It seems to be working ok although some radiators have cold spots (they don't need bled, it's possibly sludge or insufficient water pressure?). Do I replace the system ahead f it failing? With what?

My heating engineer said it's possible to add a digital control to the tank to give different room temperatures at different times of day (and presumably control the temp of the hot water). He also said the boiler itself essentially can't be repaired if it fails as the manufacturer has priced replacement parts very high to encourage people to replace the whole boiler. However the boiler is a simple mechanism that could run happily.

So I'm wondering if I should replace the system ahead of it eventually failing (with what?) Also does this mean I can lose the cold water tank. in the loft?

I have been used to combi boilers in flats so this is all new stuff. My heating engineer is not at all had sell although he does fit new systems, sso I think he's not pushing a sale although he would replace the system if it were his own house.

The TRVs are branded Danfoss.

The TRVs all seem to be working. I've done the "turn the hot ones down" game and here are the results-

Downstairs
Living room - hot across the top, cools when turned down
Dining room- heats all over, cools when turned down
Upstairs
Landing- hot across the bottom, cools when turned down
B1- warm extreme bottom right, cold elsewhere.
Bottom right becomes hotter when other radiators are turned off and there is some heat across the bottom, but the top stays cold
Ensuite towel rail- hot across the bottom, no noticeable change when other radiators are turned down
Bathroom- hot all over, cools when turned down
B2-hot all over, cools when turned down
B3- hot all over, cools when turned down

If you have any ideas I'd appreciate hearing them.

Danfoss is a major and good quality manufacturer, and may still be able to supply replacement heads (if needed) to fit your old valves.

hot across the top, cools when turned down
may be badly sludged

heats all over, cools when turned down
correct behaviour

hot across the bottom, cools when turned down
is full of air, or badly sludged.

warm extreme bottom right, cold elsewhere.
insufficient flow. Turn off ALL the others and see what happens. May improve if all rads are balanced

right becomes hotter when other radiators are turned off and there is some heat across the bottom, but the top stays cold
air, or sludge, also poor flow

hot across the bottom, no noticeable change when other radiators are turned down
Air or sludge

hot all over, cools when turned down
Correct

So it seems to me that you have a sludge problem. The air you can easily bleed out, and the balancing can be addressed later.

I would recommend starting with a simple chemical clean. If you are fond of DIY plumbing it need only cost £20 for chemicals, plus two half-days of tinkering. You will need to find a downstairs drain cock, and (probably) a small feed and expansion tank in the loft. Or a plumber or heating engineer can do it. You do not need to pay Gas Engineer prices because it is not gas work.

If you are fond of plumbing and strong, you can also take the rads into the garden and squirt a hosepipe through them.

I would also recommend fitting a magnetic system filter, to capture loosened sediment, on the Return pipe to the boiler. It is not a difficult plumbing job. Preferably fit it before doing the clean as it will stop sludge moving into the boiler.

A chemical clean is not very intensive, but it will certainly do nothing but good. If more work is needed afterwards, it will not have made things worse.

Thank you thank you thank you!