With the latest news that diesel engines are supposedly killers and the possibility that diesel cars will be more highly taxed what is the longevity of a petrol engine in terms of mileage compared to that of a diesel engine?

Unless its a heavy-stressed mini turbo petrol engine, I'd expect it to last a long time. Certainly longer without major work than a modern diesel anyway. Super diesel longevity is a bit of a myth these days.

Plenty as long as it's cared for. But petrol will be demonized soon enough.

A petrol engine is usually less stressed, as they run much lower compression. The most stressed parts are going to be the turbo, as high pressure direct injection force charged engines are capable of almost matching diesels for economy, but are lighter, less pollutive, quieter, and have similar power delivery profiles (the torque coming from the use of the turbo). Oil quality is the key to long engine life, all other things being equal.

Should be fine if looked after which is to be said for any engine.

I have had some quite high mile petrol's in the past which had no issues. 

Going back a few years (20+) I had a Nissan Bluebird 1.8 with over 220,000 miles on the clock.

I also had a couple of Ford Granada's around the same time with over 200,000 miles,