Apollo Radar chain issues I have an Apollo Radar MTB and its chain is beginning to look like it needs replacing seen as its worn. However after going to Halfords (as Apollo and Halfords appear to be together) and buying a new chain and measuring it to length and fitting it, it appears to slip off on every gear. I have set up the limit screws and adjusted the cables accordingly but to no prevail. Any tips on where I'm going wrong? Many thanks.
If by 'slip off' you mean that the chain over-rides the sprocket teeth when you apply force through the pedals it's likely that your cassette sprocket teeth are very worn.

Do you know how worn the chan was? As chains wear they elongate, the more elongated a chain gets the more stress and wear it puts on the sprocket teeth.

Check the cassette sprockets for signs of wear to determine if you need to replace the cassette. 

I always recommend getting a chain wear gauge. They are cheap (~$10 US) and easy to use. Avoiding running a worn chain substantially extends the life of the cassette and chainrings.