I agree with @robj20 , unless your mower is really old it will be fine. The problem is not using the petrol, it's storing it in the machine. Here's what Stiga say about it:
Petrol is changing to #E10 this September in the UK. But what does that mean for your STIGA lawnmower?
The ethanol content of petrol is increasing to 10% making the new E10 petrol cleaner and better for the environment. So what's the problem? Ethanol attracts water, leading to gum formation in your lawnmower's engine fuel lines and carburettor. This causes corrosion resulting in starting issues and engine damage over time.
Petrol starts to go 'stale' after 30 days from purchase, therefore we recommend adding fuel stabiliser to freshly purchased petrol. This will keep it from going stale for up to two years.
If you are storing your mower for more than 30 days - for example for winter storage - we recommend completely draining your mower of petrol to keep your engine healthy. This can be done by running the mower after you have finished mowing your lawn, until it comes to a stop meaning the petrol has been used up. For more information please consult your mower user manual or our customer service team at [email protected].