After getting lots of condensation inside overnight I discovered that the rear driver side seat was quite wet but only at the bottom of the back, not the seat itself. Took everything out of the boot and there was about an inch of water under the spare wheel!

Anyone had this problem and solved it? Checked all the boot and door seals and they all seem fine.

This sounds like the common problem with the Mk3 of water getting in through the vents under the rear bumper. If you do a search you should find lots of threads about it. It's usually fixed by removing the bumper and re-sealing the vents, after drying everything out, of course.

I have owned a 62 plate Ford Focus Titanium X, 1.0 Ecoboot since May 2021. In the last few months I noticed misting on the back windows and then found a nice pond in the wheel well. Having pumped litres of water out using a drain plunger I felt around the boot sides and under the rear passenger seat and found a bit of wetness/ damp. Looking on various forums and even change org I found this is a common problem where water leaks behind the light clusters and through the top edges of the vent-grilles on the muffle boxes which are on the back corners of the Mk3, hidden behind the bumper, below the level of the boot deck.  This is the vent system for air pressure when you shut the tailgate or doors. There's loads of You Tube videos (some with the entire interior damp upholstery removed) showing how to remove the rear bumpers, take out the four vent grilles (2 each side), take out the wadding, dry it out and reinstall the wadding and sealing the vent grilles in place to prevent the muffle box filling up with rain. You could fix it like the You Tube videos show you, but it is a right faff and sealant exposed to the elements is going to perish again. Those muffle box vents could unseal with weather, road splash and vibration and you would be back to square one. I got lucky by the sounds of it and the worst damage is a bit of water ingress and a ruffled boot wall carpet near the rear passenger seat where I pulled it out to have a feel around. I fixed the entire problem by taking out the muffle box bungs, easily reached on the underneath of the car. Water drained out of both sides. Then I pulled all the wadding out of each side (through the bung hole - yes, it is possible if you are careful, but expect wet hands / arms / armpits). Then just leave it. No bungs, no wadding, no water filling up the muffle boxes and flooding the car. Just an unmuffled self draining muffle box and and a dry wheel well.