Reads like the mounting.

The obvious answer is go contact the MOT tester and ask

I'd spray liberally some WD40 or similar water disperser spray at the area for now, then in the spring wash all the muck and salt off the underside (good practice every spring for a long vehicle life), then wire brush it all down and have a look see what you're dealing with.

Chances are painting the affected area with a rust converter then painting over that when dry and a final coat of decent waxy underbody protection will keep any further deterioration to a minimum will do, don't forget to consider the box section interior or the other side of any panel that appears rusty, worth getting to the inside of such problem areas with purpose made cavity waxes via the rather good probes that are now available for aerosol application.

Worth having a good poke nose round the rest of the car whilst there and treating anything else that looks suspicious.

Edited by gordonbennet on 15/01/2017 at 11:58

+1 for MIG welder

ps Cheapo option is gas-less.
£4 for 0.9mm gassless 0.5kg roll of welding wire.

I've just done some body repairs 1.5mm by tack welding and cooling.
Previously, I've welded a hole in a Catalytic converter, which would've been a MOT fail .

Off Topic,
Yesterday, I was welding a generator frame..
the generator was cheap because the frame was tatty.
(Honda E3200 for £80 - 1900watts,
ps this will run the welder on some settings - I didn't try them all
and a 1.5hp air compressor)

MIG welders are easy to use and it is cheap for the wire...

Liam
ps I've got a Clarke 151EN, for £70 from cash converters..
some bargains on FleaEbay , gumtree, preloved et al.

Edited by liammcl on 16/01/2017 at 22:07