Marin £25

[kingoffootball]

Unfortunately the material itself is compromised.
This means you'll need new "nipples", which come with the spokes - and take care to preserve the threads in the rim.
I'd say the rim threads tend to be ok, although you might want to clean the threads.

I have plenty of spare nipples, it's removing the old ones which is causing the issue.

 

[bikeworkshop]

Can you post a close-up pic? I thought you had those captive nipples!

 

[kingoffootball]

Can you post a close-up pic? I thought you had those captive nipples!

I'll try to take one tomorrow wwhen the light is better.

 

[kingoffootball]

tomorrow is a general term to mean at some point in the future.

 

[bikeworkshop]

On the positive side, it looks like standard spoke threading and rim eyelets - so replacements will be straightforward.

On the negative side it looks like your alloy nipples have mostly granulated.

If you can get needle moleys on the remaining bits of nipple, you might be able to break the remainder off.

You might be able to run a 2mm drill between the spoke thread and the rim eye, through the nipple alloy.
Worst case you might have to cut out (say) all the spokes on one side, to expose the remaining piece.

If you can get about 1/3 of the nipples to let go of the rim, then you'll be able to pull the rim around to expose the nipple heads in the rim well and either unscrew or break them off.

Standard nipple hole is 4mm, spoke is 2mm, so if you could find a 3 or 4mm holesaw drill bit, there may be a way forward there too.

Seems a shame to cut decent spokes