Here's the current working spec. Case: Coolmaster Elite 311 (free)
This is my old case. I will either build into it, or send it to PCSpecialist (if the postage isn't extreme). That's a service they offer. Alternatively, I could save the postage hassle and pay £20
Motherboard: ASUS Prime Z390-P (Intel 1151 socket; DDR4; USB 3.0) CPU: Intel i5-9400F (2.9GHz, 6-core)
A Ryzen 5 looks lovely, but is also about £100 more
Memory: 16Gb Corsair Vengeance (2400MHz) Graphics: 6GB nVidia Geforce GTX 1660 Super PSU: 650W. Either Corsair (from PCSpecialist) or Seasonic (if I use my existing components)
My drives I believe are functional, so I'm keeping them. However, I can't buy from PCSpecialist with no drives at all, so that's the cheapest: 256GB generic SSD. I shouldn't need to repurchase Windows, at least.
Everything else standard: cooling, onboard sound, 10/100/1000 LAN (and I have a wireless expansion card already) That spec costs either
£661 from PCSpecialist
or, buying the board, CPU, cooling, memory and GFX from https://overclockers.co.uk; plus accounting for the ~£70 value of the Seasonic PSU I could otherwise return
£621 to self-assemble.
So my current thinking is that £40 isn't enough saved for the faff and worry of doing it myself.