Hi Lawrie

Actually, I'm not sure if it applies to every tomato greenhouse in the UK but in my place the migrants got paid by the hour (at minimum rate) but only the straight eight hours a day doing just the picking, so they can - and usually do - go slow!
Us local workers (and there wasn't that many of us wanting to take on the job, so you're right when we have had so many Brits turning up and lasting only for a few days before saying they had enough - amazing I lasted out 17 years looking back!) where there were 6 local and up to 8 imported from other EU countries. The locals (including me) got to be allocated our own plot during the summer season and got paid according to the size of the plot. In other words, the bigger the plot, the bigger the wages and when you become as proficent as me after a few years of it, it actually was a very good deal. Except I had to work 6 days a week solid for 8 months. Our migrants only filled in the gaps left by unwilling local workers and they worked the flat weekly 39 hour on basic minimum hourly rate - like I'm on in the winter months - but they provided cover for sickness leaves and got paid extra for that.
If anything, towards the end the boss wouldn't pay out overtime, just hoped everyone did the work - and generally we all did. That was until the firm folded up due to massive overheads (we're one of the very few greenhouses to use heavy oil purely for heating) and poor fruit prices along with severe competition from Thanet Earth located some 25 miles away.
The greenhouse industry is quite fascinating - even has it's own charm - but when you're like me and have been involved in it for nearly two decades, it does get a little tiresome. And I don't even like tomatoes.