Welcome! I have no idea on the model your are looking at. I have just done a quick search and found this:
http://www.wiggle.co.uk/avanti-blade-1-2014/
Is that the one? If it is, then it isn't really a mountain bike. It is what is known as a hybrid. This means it offers the upright riding position and braking of an MTB whilst also offering a little of the speed of a road bike. I can't say if this is true as I have never owned a hybrid but plenty on here do and are very happy with them.
Let's look at a few of the details:
Alloy 6061 frame. This means aluminium to you and me. Light but perhaps a bit of a harsh ride. It does include a replaceable rear mech hanger. This is a nice touch for the money. Someone has thought about the design a little. This frame is matched to a steel fork which will be strong.
Alloy V Brakes. A type of rim brake. I think these are great. They offer good stopping power. You need to check the wheel rims for wear as the brakes will wear them after a lot of use.
Shimano 24 speed drivetrain. I hate that word. You don't drive it and it isn't a train, so what is it? It is of course the gearing. 24 means that the front chain set has 3 rings on it. You use the big one for going fast, the small one for climbing steep hills and the middle one for most of the time. Check for wear. Look for teeth that look like a shark's dorsal fin. On the back wheel is a cassette with 8 sprockets. Again, a cassette is a nice touch at this price. It makes replacement much easier. Look out for wear especially on the middle few sprockets. Shimano Altus is a low end mountain bike groupset. It is however quite worthy and will offer reliable, crisp gear shifts. Ask if new cables have been fitted.
Wheels. Lift each end and spin the wheel. Look for any wobbles. Try to wiggle the wheel rim from side to side. Any movement means the cones are loose. Point this out! I don't suppose the original tyres will still be on it. 28s are quite narrow for a hybrid. Fatter tyres mean more comfort so bear this in mind if the 28s (it will have writing on the side of the tyres) are still there. Check there is plenty of room between the tyres and all the frame tubes for a wider set should you want to change.
Obviously, check for general condition. Try to take someone who knows about bikes to look at it with you. Ask for a short warranty if it is a shop selling it as you will be paying a little more than if you were buying from Joe Public; like buying a car from a dealer.
As a package, not a bad bike at all.
