Morris Minor Owners Club

Yes - it could do - but if seat inserts are present then 40 thou should be ok - or just make the pockets. This is what I have done in the past - but this last head had slightly damaged seats anyway (and only cost £5) - so I tried it and it worked!

But surely if you have to sink the exhaust valves on an unleaded 940 head by 40 thou, that's going to cut away at the hardened seat and possibly 're-lead' the head?

The majority of 940 heads don't have the 'unleaded' hardened seat anyway. If you get the head machined with inserts added, you can ask for the valve seat height you need anyway.
I drive hard therefore no point relying on the 'lead memory' - I follow the option to just use an additive instead (as it cost much less than a recon head). If you're going to do 40,000 miles or more then a recon head may be worth going for price wise (or if you can't be fussed to put additive in when you fill up)

I would say overall there will be more unleaded 940 heads out there than leaded - it's just they are still on engines ! Unleaded heads can be found - and often at stupidly low prices. My head is not unleaded (neither my own or the ones on the Traveller or the Mini) and I don't use additive - but I do only low mileages in these cars

Something like 35 quid got my old 940 unleaded head as advertised on here. I was happy with the money, and I reckon it was a fair price, no point spending silly amounts on the things IMO!