Or as they are quite hardy could I just plant them out now and save myself the hassle? I am in Bournemouth

As you say they are hardy we can assume they are English ones - Calendulas? You could have sown these directly where they are to flower but don't put them straight out from a warm place to outside without hardening them off.
They do produce more than one flower per plant and should continue to do so throughout the whole summer , especially if you continue to deadhead them. Had you thought of using those trays of moulded square cells rather than individual pots? They are easier to fill with compost, take up less room and are easier to carry out to the garden.

Yes they are English and yes they were in the small cell trays at first but seemed to be outgrowing them and it was only seed compost so I moved them about 10 each into 8 inch pots with normal compost and they are still in greenhouse. Just concerned as the pots are quite deep that I will struggle getting them out with the roots and damage them so maybe I should have kept in the cell trays and used normal compost?

Perhaps, if the cells were big enough, but its too late to worry about it now! If they were sown in little cells you could have potted them on into bigger ones, and yes you would have then needed normal compost. When its time to plant them out you could gently tip the whole lot out of the 8" pots , supporting the surface of the compost with your fingers so it all doesn't land upside down...-it will be a lot easier to separate them with minimal damage than trying to dig them out with a dibber or something.

Another time it would do no harm to sow a bit later.