Shifter doesn't reach bigger gears
I went to my local bike shop and they advised me to open up the shifters and give them a good spray inside with wd40, since it could be caused by the internals getting clogged up with dirt and grease.

This worked great and I can reach all the gears now, just need a little adjustment and then I'm good to go!

The bike shop was Ralph Colemans Cycles in Taunton, very helpful staff!