I will mainly use this for on demand TV so it needs to have I player, 4OD, 5 on demand, Netflix & Now TV
What is a good one to get as I understand not all apps are always compatible on all devices, and I want it to be future proofed as well so it lasts a few years!!
When you say you will be using it for on demand TV, I presume you mean watching on the tablet itself?
If so I'd go for the largest screen and the highest resolution Android tablet you can possibly afford.
EDIT - Keep your eyes out for deals on things like the Samsung Galaxy Tab Pro or S models.
I managed to pick up the 10.5" Tab S for £299 with £50 cash back on Black Friday...now you probably won't get as good a deal as that (the £50 cash back is up at the end of the month), but the Tab Pro 10.1 (slightly smaller, non AMOLED screen, but the same screen resolution and processor) is currently £229.99 at PC World, its been slightly cheaper but its a very good tablet for that price.
EDIT 2 - This isn't to say a cheaper tablet won't do what you want it to do, if you don't want to spend the extra, the Tesco Hudl 2 is a respectable choice.
If you want to try something a bit different, how about a Linx 7, 8 or 10? (Number is screen size). Got my son the 8" model for Christmas. Runs full Windows 8.1 and all apps (Netflix etc) and as a bonus, has HDMI out should you want to watch on TV. You can easily pick up for less than £80.