Repairing expansion tank

The SOC has a very thin hairline crack in the expansion tank.

It's not a huge issue, but it does, over a few weeks, leak enough
coolant to drop from the max. to min. levels.

Given that the tanks are unobtainable, what's the best stuff to effect
some kind of repair? - I'll need something available in B&Q / Halfords,
as I need to repair it this weekend for a track session next weekend.

What's it made of?

You can sand it down and patch it with any two-part adhesive from
the car-spares shop, designed for the purpose, and it will still
leak again as soon as it gets hot.

Any experts in plastic welding in the house?

I dunno about expert, but I've welded many plastic parts and the main thing
is to use the correct rods. There's usually an indication of which plastic
an item is made of (stamped/molded on it somewhere) and then you know which
rods to use.

Tack the parts with a soldering iron and form a "V" which is then filled
with the plastic rods. Easy peasy.

Gently run a soldering iron along it?

It did cross my mind, but I'm a bit worried about doing that with
yellowed, brittle plastic - I may well make it worse.

Rough the line of the crack up with a light file or abrasive paper then fill
it in with Araldite (old type not rapid).
Gently heat for a short period. emphasis on gently!
I have had success with that.

Can you get a stainless metal one made to order? You can get 'em for
most cars?

Swap it for one from another car? I can't see a repair being particularly
effective TBH.

Yes, they're really that easy to get hold of. Most are in just as poor a
state as mine.

I meant another make or model. As long as it'll pysically fit, I can't see
much of a problem with it. Hose barbs etc. can be made to fit with adaptors.
Has it got a level sensor?

Yes, it has a level sensor.

Not sure if I would get another one to fit - it's an odd shape.

For now, I've slapped a bit of Plastic Padding 'Leak Stop' on it - if
nothing else it'll stop the cracks from growing whilst I try and source
a decent used one.

I can't see any other solution.

Take a picture. I'd not be surprised if a very similar one wasn't used in
another car somewhere...

As it's a SOC, just plaster the tank with fibreglass resin & tissue.