Always get a total price for the job rather than a day rate.
Ask for a detailed list of what's included.
Ask them to think if there's anything that might come up as extra.
As what hours they work. Should be 8-5.
Probably best if you get Poles or other nationalities. Brits in my experience will turn up early sometimes, but mostly around 9 or so, and will indeed go off for 4 hours to B and Q for materials, will have a lot of cups of tea, and will tell you their life story. Get guys who don't speak much English and then they can't do that ...
Seriously, though, you're better off with a total price for the job.
And just don't make them tea and talk to them. Go out. Go anywhere. Say you're ill. Deliberately don't talk to them. If they're on a day rate, it's crazy to sit and talk to them - they won't say no because they think you're tacitly ok with the waste of time, because you're doing it.
But as I say, don't get them on a day rate. They'll do it much quicker and be more motivated on a price. How hard they work/how quick they do it defines their profit.
Depending on the length of the job, give them some up front, some halfway, some at the end. Never ever give them the final payment until they've done it all. And if they say there are extras, just calmly check through what these are - if they are legitimately extra, just pay and don't complain - there's such a spectrum of what is extra and not, but you will usually know if it's fair or not.