My father has a defender and lights work fine on dipped but as soon as you flick them to high only the side lights stay on. I have tried another switch unit but the problem is still there. The hi beam light on the dash works but thats about it. I'm crap with the sparky stuff. Also (if) someone were to put 100w bulbs in what other measures would need to be taken so it would not overload the system or will the wiring handle the increase in demand?

Simple test. Have you looked at the fuses? My no. 12 fuse on my ninety blew and i had similar light problems.
Not 100% sure about if you change the bulbs, and not sure if that wouldnt make it illegal for road use.
Any suggestions anyone else?

You must fit relays or you will melt the wiring, whether the bulbs are illegal or not. I have relays on the dip and main of my Defender to protect the switches and give greater voltage at the bulbs.

The sides are on a different circuit and when you flip to high it kills low, so the problem will be in the the high beam circuit, could be a loose wire on a switch or relay or bad earth, or that the fuses for BOTH lights have popped which is unlikly. If the blue dash light is coming on then I would expect that its a relay issue.

100W bulbs would be illegal for road use, if you want brighter get those ultra bright white things you can get, also using the 100w could lead to wiring problems due to the increased current flow.