thanks. Caveoftreasures & goodolddays for the kind words. 
 I de-soldered all the large blobs of solder that was on all the plugs and some of the tracks and then re-soldered all of them properly. It makes it so much easier when you have the right tools for the job like an adjustable temperature soldering station and a good quality de-soldering tool, both of which are now over 10 years old. Also for this board i used a good quality solder, 0.7mm, I know it's a bit thin but i have found it gives me better flow control when soldering.
I have found that this type of board has some bad design issues. a lot of the high current tracks are on the top of the pcb and rely on solder throughs to get the power to the plugs and fuse holders which are soldered on the bottom of the board. This wouldn't be a problem if the holes for the plugs where bigger to allow solder to flow through the whole joint and to the top side of the pcb and attach it self between the top track and pins that are being soldered to the board, but the holes are so close to the size of the pins and don't allow this to happen. It's not so bad with the fuse holders as you can solder them from the top and bottom. That is why there were so many hacks on this board, this is a common hack done on these boards because of the design fault.
I way I got around this with the plugs was by first soldering all the pins on the bottom of the board and then gently prising the plastic on the plugs up (with a small flat screw driver) so i had enough room to solder the pins to the tracks on the top of the board (being extra careful not to break the black plastic as it is quite brittle), then pushing the plastic back down again once this was done.
With the G.I relay I had to raise it slightly so i could get the iron tip underneath it and flow some solder onto the pins and pcb. ( I used a long thin iron tip for this)
The end result was good strong solder joints and noticeably brighter G.I. & insert lighting throughout the playfield & back box.