Monty's trying to fettle his 2004 3speed Brommie...

[Dogtrousers]

Perhaps the OP needs to contact someone like Brilliant Bikes to see if they've come across something like this previously. The chainring doesn't look outrageously worn, but even if it was, if the pitch of a new chain was correct, it ought to fit round the chainring even if it ran rough or jumped under load. Back in the days of 5 speed derailleurs I've run chains with chainwheels with far worse appearance than the OP's pics.
In any case, the pitch ought to be the same, it's the width that changed, and that ought to either fit, or not. It's baffling.

Is there any remote and bizarre possibility that the new chain is some kind of knock -off made to inaccurate standards? It sounds unlikely, but who knows nowadays.

These points are why I was kind of tentative with my chain "stretch" theory. You should be able to fit a new chain onto a worn chainring, even if (as you say) it starts jumping when you put it under load.

If @MontyVeda has a chain checker it may be worth seeing what it has to say about the 2 chains. Maybe it's very cold in Lancaster and the new one has contracted

 

[Fastpedaller]

These points are why I was kind of tentative with my chain "stretch" theory. You should be able to fit a new chain onto a worn chainring, even if (as you say) it starts jumping when you put it under load.

If @MontyVeda has a chain checker it may be worth seeing what it has to say about the 2 chains. Maybe it's very cold in Lancaster and the new one has contracted

Maybe Shimano have tried to re-introduce the 10mm pitch chain they tried to sell (was it the 80's?)

 

[MontyVeda]

These points are why I was kind of tentative with my chain "stretch" theory. You should be able to fit a new chain onto a worn chainring, even if (as you say) it starts jumping when you put it under load.

If @MontyVeda has a chain checker it may be worth seeing what it has to say about the 2 chains. Maybe it's very cold in Lancaster and the new one has contracted

That's what i was thinking... a new chain should fit onto a worn ring, but this one clearly doesn't.
I do have a chain checker but i haven't checked the new or old chains with it yet.

Not going to be much progress until the weekend now but it would be nice to get it on the road next week.