I'm not familiar with Halfords. Is it a bike shop or a department store?
The Aggressor is an entry level bike that comes with coresponding entry level parts, including the wheels. I am guessing that you are not a featherweight and/or you have been riding the bike like you would expect a mountain bike should be ridden. I am also guessing that when you replaced the axle and rear wheel you bought less expensive pieces that were a quality match for the original stuff.
You are going to have to buy a better quality wheel. You may be able to buy a stonger axle, but it may just be better to buck up and get a higher end wheel. If you try to replace the axle again, make sure you get a good chrome-moly axle. You may have to get new cones and bearings to make it work with the rest of your wheel. If you get e new wheel, make sure it is a good quality wheel, like a Shimano LX or better hub and correspondingly better quality rim, etc. The bike lists for about $300.00. If you want to get a wheel that will last, you will have to spend over $100.00 for the wheel. Don't bother with a $50 replacement wheel, it won't last unless you are under 120lbs, only ride on the street, and don't do jumps higher than curbs.