Any bike folk about?

Another vote for a hybrid. Beware there are hipster bikes in the hybrid category like Pashley brand which are pretty and well made, but weigh a ton.

You want ideally an aluminium or minimum cromoly frame. Avoid steel or high tens (steel)
Disc brakes are less hassle than rim brakes . Hub brakes are less hassle again.
Hub gears are lowest maintenance, but you get less gears.

Think about accessories too - do you need lights? If so dynamo lights are the simplest, just fit and forget ,( but make sure rear light stays on when you stop)
Do you need a rack or may do in the future (check frame can be fitted with rack)
Mudguards are definetly worth getting too.
Also, as pps have suggested it is worth upgrading your tyres to puncture proof, and making sure you have a saddle that fits.

Also consider a pump, basic tool set, and a book on basic maintenance.

Iirc Evans will order a bike into store with no obligation to buy it. You might have to pay a deposit but it should be refunded if bike no good.

If the op is nervous on a bike not sure I'd recommend drop handle bars. I found my cyclo cross twitchy compared to a mtb.

The problem is you can't really tell if a bike will be comfortable for you without actually riding it. There is so much variety in frame size and design. Could you not go to another retailer (or two) test ride some of theirs, but then still order online?

Yes a hybrid will be much better than a mtb. Mtb are hard work to pedal on roads

Specialised do a great hybrid. Think its called the vita. My friend has one. But it needs to be something which feels okay from a fit point of view. Remember the saddle can be changed.

What about a cyclocross bike? Its like a mixture of road and bmx but not a hybrid iyswim?
Cyclocross is an off road sport that gets very muddy, so you'd have the chunky tyres that you like but lighter and easier to manage with dropped bars like a road bike.

You could always get the bike resprayed purple!! Or customise it with a purple saddle and bar tape and bottle cage etc.

Size of the bike you need depends on your height and your reach, maybe not one marketed as female. Check out Wiggle Size Guide

When you say off road cycle tracks do you mean tarmacked or MTB black routes?

You are most likely after a hybrid, which will be ideally suited to off road cycle trails and a bit of town riding. Evans will have a good range available. You don't necessarily need a women's specific fit, unless you are set on a step through (ie, not a straight across crossbar at the top of the frame). Women's specific geometries will most likely come with slightly more female friendly saddles though.

Don't buy anything with suspension - will just add weight and be totally useless. Consider upgrading your tyres to Schwalbe Marathon Pluses - I have cycled thousands of miles with those tyres and never had a P*ncture.

Good luck and have fun!