I recently bought an 04 reg 2.2 HDi exec Estate, in black, from an advert at the roadside, like a lay-by (I know, I know)

I needed a car and had seen this one on my way to work for a few days so called the guy up to look at it. He met me over there as he was out working elsewhere - he said he put the car there to get more attention for the sale.
It was advertised as low mileage (52,000) and looks and drives very nicely. There was quite a few stone chips touched-in on the front and the guy said it happened last winter by following a grit lorry and some grit fell onto the road.
I was pleased overall with the car and we shook on £6,150.00 and off I went apart from he said the service books were at his girlfriends house and he'd post them on as he forgot to bring them but he said there was only 4 stamps in the books anyway because of low mileage but I had the reg document ok.
A few weeks went by and a fella pulled into Asda's car park next to me and was looking at the car. When he got out he approached me and said it used to be his an how was it running now? I said it's ok but why ask? he said he used to run it as an Airport car for his taxi firm which he closed down last year. He said when he sold it on it was showing 160,000 miles and a local guy bought it but sold it because his wife had put Petrol in it twice!.
It seems this guy I bought it from is a trader and hadn't registered it himself so left the last keeper on the form. The mobile number I had for this guy now doesn't work.
I had the car valued by 2 places with the correct miles (we guessed now about 170,000) and they both said about £3,000.00 I'm also having trouble with an "ANTI POLUTION" message and haven't got a clue if it needs a cambelt yet or what? also the key for the doors is different to the ignition key? is that right? the guy said Peugeot do this to make it harder to steal?
I also don't know what to do about the mileage and whether I can sell it knowing the mileage is wrong? can it be changed? but what to? I'm thinking of asking Peugeot to quote for bringing the car up to date with services.
Any advice welcome.