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The middle / bridge tone pot on my strat had suddenly decided to act like a volume pot and my middle pickup sounds weak as fook, what's gone wrong?

You took too much! There's no turning back!
Seriously though, if you or a friend own a multimeter I would check your pickups and pots and see if the values seem normal. Maybe somebody more clever with this stuff will chime in.

I also don't know what is wrong with your guitar.

Given that the pickup is kinda working but the tone control isn't [as intended], my guess would be dirty/bad pot, loose connection on pots/switch/capacitor, or maybe an erroneous connection caused by messy wires/a knob twisting round in it's mounting hole. By all means check the pickup over with a meter, but if that yields no smoking gun give the harness a once over against a wiring diagram while giving each of the switch/pot connections a ruddy good wriggle to make sure they're solid, then nuke it with contact cleaner from orbit to be sure.

Had a good dig about under the hood. The pot acts as it should when using the bridge pickup on its own, and like a volume when the middle is selected. All the connections seem to be solid, I put new pickup covers on the other day could I have done some damage to the pickup it's self? Unfortunately I don't have a multi meter (and if I did I ain't got a clue how to use one).

dezb1 wrote:Had a good dig about under the hood. The pot acts as it should when using the bridge pickup on its own, and like a volume when the middle is selected. All the connections seem to be solid, I put new pickup covers on the other day could I have done some damage to the pickup it's self? Unfortunately I don't have a multi meter (and if I did I ain't got a clue how to use one).

That could explain things. If you've accidentally caused a break in the wiring or disturbed a badly solder joint, it'll be like there's a gap connected by a very small value capacitor. The tone control introduces a larger cap progressively and progressively more signal is lost across the high impedance "small capacitor" of the gap. That should be easy to measure with a meter.

My 2008 USA Strat came from the factory with a bad solder joint on the bridge pickup that got worse with age but was more noticeable with some amps than others - a really high impedance input showed a small drop in volume but a lower impedance solid-state amp showed a huge drop compared to the other pickups. I got it fixed under warranty.

It isn't stock wiring to begin with, because strats are wired with only neck and middle tone controls, so you can get the shrill ear-piercing bridge tonez.

Although wiring to the bridge is a good mod.