£8k sounds ridiclous money for a car in that condition! With that kind of milage, you shouldn't be paying any more than £6k... There's cars out there for under £6k with less miles than that even!
The Petrol Vs Diesel debate... Well...
The petrol you will average around 30mpg as long as you're not caning it everywhere. The diesel around 45mpg. So at current fuel prices, that's about 12pence per mile fuel in the diesel, and about 18pence per mile roughly in the petrol. Based on your annual milage estimation of 10,000 you're looking at the Petrol costing an additional £600 per year in fuel over the diesel...
Both are good engines, the petrol perhaps slightly nicer to drive, but the diesel is very very strong, and very good for an oil burner. Pure performance terms, the diesel is a tiny bit slower, not much, but the diesel will have more mid range shove than the petrol (though again not by much as the petrol is turbocharged too).
It's hard to recommend either the petrol or the diesel outright to you based on your situation... How long are you going to keep the car? It's really a case of how much more is the diesel than the petrol, Vs the length of time you're going to keep it... Diesels of the same vintage and condition can often fetch £1500-2000 more, and if this is the case, and you will keep it for 3 years or less, the the petrol will cost you less in the long run. If you're gonna keep it for a long time, and/or it's only £1k or less price premium, the diesel will save you money in the long run.
For the record, if it was me buying it I've have no choice but to go for the diesel, but then I do 25k+ miles per year (a mix of work and leisure), so a petrol would cost me about £1500 per year extra, just in fuel!!!
Incidentally, both can be "remapped" with good results. The diesel goes to about 200bhp and the petrol to about 260bhp straight off, both with improved economy. They're serious Q cars (hence why popular with the cops), and a well driven Octavia vRS will be a hard car to keep up with.
Oh, and as for the Octavia itself... Great cars... Definitely get one over the competition!