"Alan Holmes" wrote
The sweetcorn and some of the tomatoes are getting large, and I
worry about leaving them in the greenhouse too long, so when
will be the earliest I can put them in the ground?

When all chance of frost is passed in your garden. :-)
Old gardeners around here used to say after Chelsea i.e. about 1st June.
That said we had a bad grass frost on the 9th June a couple of years back
but it only affected our allotments nothing to show at home.

--
Regards
Bob
In Runnymede, 17 miles West of London