My front right side floor panel looks like this. So I'm trying to decide whether to weld in a patch or fit a new floor pan.
Does anyone have experience of cutting out the old pan and fitting a new one, was it straightforward or tricky?
I've done both. On the LHS (Drivers) the floor plan was rusted pretty much front to back, so I replaced the entire thing with Mercedes parts. On the RHS the damage was much more localized - it looked similar to yours.
I'd say that if the rust gets close to either the cross member, or the short 'beam' that runs back from the firewall, you should replace the entire panel - especially if the area behind or under the cross member is affected If it's anything less than that I think a patch is ok (that's the criteria I used).
If you do decide to do the complete panel, you defintely need to remove the cross-member (even if it isn't rusted out). Also, you need to make sure you have a good tool for cutting out spot welds - seemed like there were a thousand of them. The only area that was a real pain was where the panel attaches to the rear crossmember (at the end of the rocker). You need to remove the subframe mount.
Though it looks like a patch is the easiest approach for you, I thought I'd post a few photos of each side of mine, so you can compare the work involved...
PS Nice post of the MOT guide - even though I live in a US state that has almost not inspection, I still use my UK MOT experience as a guide when repairing/maintaining my vehicles. I always thought it was pretty fair, and kept things safe without being unreasonable.