So the parts are purchased, when the last ones arrive next week I'll build it in secret to make sure it all works then dismantle again ready for Christmas. My wife wanted to buy my son a monitor as he currently games using a PS4 on a TV so if we ignore that then the total came out to £385.
I really wanted to go with Ryzen but due to stock/prices I just couldn't justify the cost. I'll also be able to claim a free Assassin's Creed game with the MSI motherboard for him. I had some parts already which helped knock off a few quid.
If anyone cares the the final specs/prices were:
Intel i3 10100F
£74.39
1st Player Rainbow R3 Case
£24.19
Be Quiet! System Power 9 500W PSU
£39.99
MSI MAG B460M BAZOOKA Motherboard
£79.99
2x8GB Corsair Vengeance LPX DDR4 3000MHz
£47.70
GTX 980ti
£100.00
120gb ssd (part of another purchase I made)
£10.00
500GB 2.5" (had from existing laptop)
£0.00
Win10 free/unregistered
£0.00
Wifi USB adapter (from the 'box of stuff' under the bed)
£0.00
Zalman F3 120mm x 2
£8.50
Total £384.76
Overall I'm pretty happy with that for the price