I think you are totally over thinking this.
I suppose it depends how much red you have, I assume you were low on fuel hence the desperate need to use red? First the system will not be dyed red from having 20 litres of red in the tank.
Get it going and drive it about, drive later on at night when the VOSA inspectors are all home drinking tea and just drive it till the fuel light comes on, go to the garage and put 25 litres in it, drive that off and fill again with 25 litres, each time run it till it's on reserve and then brim the tank. I stuck 20 litres of red in my 90 about 6 years ago as I was low on fuel and working on a friends farm using the 90 all day. I left with the fuel light on and filled it up at a local garage I was changing the fuel filter a couple of weeks later and there wasn't even the slightest tinge of pink.
Keep the receipt and all the MOT related paperwork and all sorts of receipts you have to prove the situation. If stopped just say, "Please see this wad of paperwork, it was off road for months, used it with some red, it was on the empty light and I bought this amount of fuel etc. etc. MOT will prove miles travelled".
If you are really worried, after your MOT take it onto a motorway or something and just drive till it's empty, refuel as said above in 25 litre fills and burn it all off in an afternoon. If you are caught on the same say as a new MOT after being off road you have a really good viable excuse.