I disagree with the “get what you pay for” sentiments a little bit:
My son has a carrerra bike, it cost a fair bit, and it has been nothing but problems, and has cost me a lot of money to fix over the past 18 months that he has had the bike.
Generally bikes do cost more to repair the more they cost, so a £1000 bike will cost more to service than a £500 bike (because the spare parts are fancier), but a £150 bike frequently won't be serviceable at all - for example on a cheaper bike you might find that the rear gears wear out the hub as well, and then because it's not at all economic to replace a hub, you'll find a rear wheel is £60, the gears £30, the chain, £20, and then they'll probably fit new cables, labour, etc., and you've spent £150.
Carrera bikes are fine but Halfords often don't maintain them well, and charge lots of unnecessary money for repair. It's worth finding a local repair man who will charge much less.
A lot of the time it's just incompetent minimum wage mechanics - on cheaper bikes you can get a new chain for £10, brake pads for £4, gear cables for £4, etc., and the actual cost of maintenance is very low, but when you go through a bike shop that knows how to run up the bill, that will easily turn into £20, £10, £10, plus labour, etc.
And bikes do need a fair bit of minor maintenance, i.e. cleaning the chain, adjusting cables a bit, which is really quite minor, but if you pay someone to do it then it will tend to cost a lot if they know how to charge....