IMHO C2 is not unreasonable. If the cover were displaced, which could happen quite easily, it becomes a C1 instantly.

Surely, there should be a cover over the switches. A falling object (it is a storage cupboard) could kill a circuit or the whole lot at an inconvenient time. Probably C3 on its own.

The solution is to repair the CU. I am unclear what is wrong with the LH screw.

If the screw holes are damaged, they may be repairable. Have a look for "Back Box Saver". I have made similar devices - just turn a top hat-shaped cylinder of brass of a suitable dimension. Tap a thread. Hold in place with some heat shrink.

As others have said, do not open the CU unless you know what you are doing. You only get electrocuted once!

I use captive nuts as long as some metal is left for it to clip onto 

the 'spire' nuts are sometimes good on backboxes with side lugs - though sometimes getting too close to an edge is awkward.

I have seen folk drill right out the back and thread into the back of the box, or even into whatever is beyond.
There are inserts that can be staked into plastic pillars, or into modesty blocks to make Right angle fixings

as well as rivet nuts for thin materials like sheet metal, sheet fibreglass and tufnol,

However, all of this, much like Chris's 'just spin a part up in your lathe' assumes that you are (or have a friend) suitably "tooled up" to do this. Those who have the facility to make up control panels etc do, but the average fitter type domestic installer probably isn't that familiar, and charging for a complete new part is less risk to them.
Mike.

mapj1 said:

However, all of this, much like Chris's 'just spin a part up in your lathe' assumes that you are (or have a friend) suitably "tooled up" to do this. Those who have the facility to make up control panels etc do, but the average fitter type domestic installer probably isn't that familiar, and charging for a complete new part is less risk to them

It's not just a case of being 'tooled up'.  Most general electricians perhaps would be advised not to take on the risk of 'owning' modification of a product by their insurer, or perhaps industry guidance.

If you carry out repairs such as this ('modifications'), can you vouch that they are no less safe than the original product? If anything goes wrong, the OEM could well have an 'escape route' ...

Yes, I know this seems petty, but it's not DIY, and a claim (or in extreme cases prosecution) could result. Would the electrician's insurer cover them for this? And despite the best intentions of the homeowner / landlord looking to save on repairs and maintenance, they might not have a say in the matter and it may be their insurers (or perhaps a regulator or prosecutor, if someone gets hurt or an extreme event occurs) that actually proceeds against the electrician.

It's not an easy topic ... the automotive industry, for example, is often making noises about 'pattern parts' and certain repairs. It seems to be the way the world is going.

gkenyon said:

If you carry out repairs such as this ('modifications'), can you vouch that they are no less safe than the original product?

That's a very fair point. I do not see how it can be. When the screws are tight and the cover is closed, all is well, but if the cover is opened, will the nut stay in place, and can it be used again?

If I were going to make a repair for a customer, and charge a reasonable hourly rate, it may not be cost effective.

So when a Back Box Saver is used, where does potential liability lie? With the electrician, or with its manufacturer?

There is a role for make do and mend, but legislation may get in the way. What a pity!

gkenyon said:

If you carry out repairs such as this ('modifications'), can you vouch that they are no less safe than the original product?

That's a very fair point. I do not see how it can be. When the screws are tight and the cover is closed, all is well, but if the cover is opened, will the nut stay in place, and can it be used again?

If I were going to make a repair for a customer, and charge a reasonable hourly rate, it may not be cost effective.

So when a Back Box Saver is used, where does potential liability lie? With the electrician, or with its manufacturer?

There is a role for make do and mend, but legislation may get in the way. What a pity!