Apparently petrol 'goes off' in some way and this can cause problems with
some things, but I've had petrol stored for a few years and it's been
apparently ok.

What happens is that the fuel can absorb water from the air if the
container isn't full, but with such small anounts this is unlikely to be a
problem -mostly when you leave a car standing for ages, it absorby water
from the air.
Also some of the more volatile parts can evaporate after a while and leave
a light glazing, like clear laquer on surfaces. Also hardly a problem in a
5 litre container. Even less so with a mower motor.


And the biggie.....Are they cheaper to run than an electric?
I don't think the relative running costs are really significant.

Petrol mowers are more powerful so good for larger mowers and larger
lawns, but they probably require more maintenance, are noiser and
smellier to use, heavier and more expensive.

Ok...The smell would be an issue...But I don't think that the noise
level
would be...Its only for an hour a week or so

The noise issue is an issue for me using it. I specifically looked for an
electric mower that was quieter than average to make it more pleasant
(less unpleasant?) cutting the lawn.

Some of the new motor mowers are very quiet - our neighbour has one and I
thought it was just a louder electric one. Worth checking out if you need a
motor mower. I don't find that the motor mowers are very smelly, except
when I spill fuel topping it up. 2-stroke strimmers on the other hand....

Maintenace-wise, a petrol mower probably needs more running maintenance
than an elecrtic one butat least you can likely fix it yourself if you know
a bit of basics about engines. An electric one may need a serious trip to
the dealer if it goes wrong. All the usual things like cleaning/sharpening
you have to do to both, so the only real practical difference I found is
you should properly overwinter the motor mower(change oil, clean plug etc.)
which only takes about an hour or so each year, whereas you can just stuff
the electric one away in the shed.
Tim.