What are the implications likely to be for the MOT station?

If anyone from within the industry could give me some feedback on this I'd be very appreciative.

Basically the story is this. Less than a month ago I purchased a van from eBay. I paid up front and collected from a third party - not the seller. Straight away there were big problems and within 30 miles of collecting vehicle it had a total electrical failure on M6 at night which resulted in police towing vehicle away. So straight off I was £150 out of pocket and also needed repairs to make vehicle drivable which I paid for. 

At the same time I advised seller that I wanted to return vehicle under Sale of Goods act - the van wasn't fit for purpose. He basically wasn't interested and was willing to go through small claims court if I wanted to get anywhere.

So, while this is in the works I've been investigating the vehicle further. It came with 12 months MOT which is about a month old as I type this. While cleaning under front seats yesterday I found a huge amount of corrosion and after taking out front mat found it's totally rotten and riddled with holes around seat mounts, hand brake lever mount, seat belt stalk mounts etc etc as well as inner wheel arch. It's severe, obviously hasn't happened overnight and makes me wonder how the MOT tester missed it all. I'm sure a tap with a testers hammer would see the whole thing fall apart. 

With what ive come across I feel that the MOT should never have been issued and also backs up my case that the vehicle isn't at all roadworthy. To further strengthen my case I feel that I should make a complaint to VOSA and have a full retest which the garage testing would obviously be invited to attend. 

My question is - if the testing garage is found guilty of issuing a certificate on a vehicle that should never have passed an MOT how serious is their penalty likely to be? Is it a hefty fine? Suspended licence or just a warning/slap on the wrist?