Rusted sub frame

change the sub frame.
ask your brakers yard for one

in UK i can get one for 50-60 pounds

Year make and model? If you have a front wheel drive? you have a serious frame and mount problem. My guess would be the frame is rusted away. Probably should not be driving this car.

go to a wreckers that handles your make of vehicle and buy that section complete ( rust free unit)
drill out the spot welds where the rust unit is attached to the body sub frame
do the same for the new section
position the new section in place against the sub frame and you have 2 options her
either weld the parts back together ( best get a qualified welder to do this)
or drill holes and use grade 8 bolts to bolt the sections together
remember that it is an integral part of the sub-frame so it has to be done properly for safety

support car place jack stands under sub frame. using an engine support to hold motor in place, disconnect anything that is attached to sub frame, ex: motor mounts. then unbolt sub frame bolts(4) and raise car to remove sub frame. note sub frame is heavy.

There's a 15mm bolt that has to be taken out under the fender on the driver's side.

How can a 13 year old sub-frame, have rusted out?
You could remove it,media blast it,and cut out rusted sections and weld in new.
You could get a used one from a salvage yard.

at the center bottom of the engine on the sub-frame the engine sits on, it is the one closest to the radiator

yes there is a recall on that!! model and you need to take it too a dealership to effect repairs

You may need to get rid of it. If the frame is rusted, it's only going to get worse. If the motor mount is not sitting just right, it will only get worse too. I would either sell it as is or have a welder to look at it to see if it's repairable.

yes a subframe from 91-94 will work.