Thought I would post a follow up to say I got some new heat sinks today to go with the MOSFET's so was finally able to solder them onto the board.

Bingo magnets are working (despite having a muppet soldering them - me)!

I did wonder tho since there are 3 magnets, is it an all or nothing thing, In other words if one magnet doesn't work then neither will the other 2? Because it's hard to determine just by holding a ball over the top of the playfield if they are all working. I'm still wondering whether my dodgy soldering was good on all of them or maybe just one or two.