Well I have heard tell of certain petrol stations watering down their fuel with various things to make a litre stretch further (and therefore sell the same real amount for more, or lower the apparent prices with no penalty). Was a big stink about it in my uni town while i was there. Thought that sort of thing was all cleared up though.
This may be a problem of that ilk (IIRC it wasn't exactly water, but some cheaper petrol-type substance that doesnt make your engine work anywhere near as well but mixes and doesnt cause a
lot of damage)... but seems a bit of an extreme case. Wouldn't everyone's cars be spluttering out of that station if it was 100% the fuel's fault? Perhaps something's dodgy and borderline with your engine and it's just the "cheaper" petrol has combustion characteristics that aren't as forgiving as, say, Esso's that has lots of funny additives in.
After all if it's a 1043, they're supposed to be able to run on petrol that's like scooter juice compared to the bigger engines (especially the gt/g40). They're happy on anything down to 91 octane, which you can't even buy in this country any more (i think the leaded equivalent would have been 2-star? - which is why they seem so perky on "normal" 95 octane unleaded, which is more like 4-star), and below that they work but "at reduced power"... no mention in the manual of it stalling...
Could be a matter of additives doing stuff, you mentioned that it not only went funny at the supermarket (likely to be the cheapest, unaltered raw petrol) but at BP as well (who probably put things in the normal fuel as well as the plain wierd lookin' "Ultimate"). Esso likely as not put additives in, but different ones. Maybe some more compatible with your engine than others. In any case, give it a proper check over (plugs, leads, air/oil filters, blockages in the intake, coolant & oil levels, run some injector cleaner in the tank etc) and if there's nothing else that you (or a pro mechanic if its real bad) can find, and you still think it's the petrol, get in touch with trading standards quickly and see if there's been other complaints from drivers using that station, maybe through the local newspapers.
I'm still jealous of anyone who gets 115 from their 1043.... what petrol was that again (and which mountain?!). Seriously, you're up on the rev limiter there in 4th (for 4-speed or economy box... 5th for close-ratio), and only pulling about 80% of max power any more
(more doable in eco-5th, i suppose..).
Even on a very good day, getting it into the indicated 150kmh range is a triumph... but, topping out at 55, thats gotta hurt
Even with the ignition and air filter troubles I've had before, performance wasn't hurt THAT badly.. once it was running past 2000rpm, it was quite happy and would go up to the motorway limit alright. 55mph is a sign you're effectively on about one and a half cylinders (say... 1/3 of potential output.... though that's not a peak power speed in any gear, except 3rd in a close-ratio, so maybe 2 cylinders/one-half power? still sucks in any case)
try a different garage perhaps? (maybe one close to that dodgy BP petrol station?)
