I would be surprised if there's much tail-off in efficiency from a solar-thermal vacuum tube.
Being 25 years old isn't a reason to replace the radiation collector part of the installation.
Any wear and tear will be in the pump, valves and expansion-vessel on the water/glycol circuit.

You don't say what proportion of the collector array is accounted for by the 4 broken ones.
Was it originally a 20-tube or 30-tube array?

You could move the intact tubes along so that the 4 missing positions are at the end where the water enters the manifold at the top.

That will prevent heat escaping from unoccupied ports at the hot-end.

Did the builder break just the glass envelopes, or damage the inner copper tubes too?

If the copper inner cylinder has been compromised, then its internal fluid may have escaped, rendering it useless.
But you'd expect to see visible signs of mechanical damage if that's the case.

I previously gave Stoves and Solar as a source of spares.
There's also Horisun in Rutland.

Either of those installers may know of a site closer to you where someone else has decommissioned a solar-thermal array and has second-hand tubes doing nothing.

@transparent it’s a 20 tube panel and at least 4 tubes damaged. They don’t like being moved and today whilst talking to jack at stoves and solar that I noticed the nipple was broken off another tube. I’m not sure that they have retained their efficiency, it wasn’t uncommon for my water to be up in the high 60s if not higher and even before the damage I was deliberating whether it might be time for new tubes.  Anyway jack is sending me a set of tubes so that’s it back as new outside.  

in terms of other matters relating to solar of the electrical kind.  Is it advantageous fitting less higher powered panels than the 10 or so I can install to my spare string.  This to me sounds logical but you know more than I and I’m guessing there is a reason folk fit loads of panels rather than a few. 

Posted by: @david999

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Is it advantageous fitting less higher powered panels than the 10 or so I can install to my spare string.  This to me sounds logical but you know more than I and I’m guessing there is a reason folk fit loads of panels rather than a few. 

The answer to that question can be seen here in the topic about adding more solar panels 😎 

@transparent thats me removed all my evacuated tubes in preparation for the new ones. A couple of questions if I could please. 

1. I noticed that random tubes were cold where others hot, does that mean one or the other are faulty. 

2. now that I have the opportunity is it worthwhile painting under the solar panel area white, they originally had reflectors on them but very fragile and blew off years ago. 

many thanks guys. 

@david999

Have you thought of using kitchen foil as a reflector?

@derek-m it’s a steel box section roof so it’s a possibility that I could make a good job glueing that on I guess.  Good idea.

It's difficult to keep the undersides of the vacuum tubes clean, let alone any reflecting surface below it.
The lower the angle of the tubes, the worse the problem.
Moss-type debris tends to stick to the underside of the glass.

I would suggest giving this some thought before sticking down aluminium foil.

Either the reflecting surface needs to be removable for cleaning, or else designed to be cleaned easily whilst in-situ.

A pressure washer would do the job... but I doubt that aluminium foil would stand up to it.

I periodically remove my vacuum tubes and wash the underside with a pan-scourer in soapy water.

@transparent I just spun my tubes to clean them, probably why they came out so easy. I have good access to clean the garage roof, and cleaning the foil straight forward but uncertain if it’s worth the trouble. But shall think it over. 

any idea what tubes could be faulty, the hot ones or cold ones. I have asked jack at stoves and solar but not replied yet. 

Here we have hazy-sun, manifold reporting 51°C
and the glass is below the temperature of my hand, so I guess you'd describe them as 'cold'

I wonder if your hot tubes are due to cracked glass (loss of vacuum) or damaged copper insert?

... and then I wonder if their failure is linked to them being spun.

@transparent I gave them copious coating of the thermal grease and a wee squirt of silicone spray, so they have always been free. My thoughts are that the vacuum has deteriorated, but I’m not unhappy considering the age. I think a rub with the silicone spray helped keep them clean but who knows it might have impacted on the integrity of the glass. But I think they were just done if I’m honest. Googling glue to attach the foil to my metal roof at the minute, a spray I think of some sort.

If air has replaced the vacuum, then those tubes will weigh more.

Put them on kitchen scales and see if there's a difference.