>>>Is there a limit to the length of time before I take it back though?
Yes, but it's not specified!
The legislation only gives you the right to receive a full refund if a fault is discovered, and reported to the vendor, within 'a reasonable period'. Most retailers regard that as being around 1 month but only a court could decide what is, or isn't, reasonable. (For example, a woman purchased a car during the winter months and didn't find out until the following summer that the air conditioning didn't work. A court ruled that it was still within a 'reasonable period', as there was no reason for her to test the air conditioning before then).
After the 'reasonable period' you lose your right to a full refund but you retain the right to receive an exchange or repair of a defective item for up to 6 years in most cases as long as the defect was there at the time of purchase. (e.g. poor quality materials were used, or there had been poor workmanship). For the first 6 months of that period there's usually an automatic assumption that the cause of any problem was such an 'inherent fault' but thereafter it's up to the purchaser to prove it (or, at least, to show that an inherent fault was the most likely cause of the problem).
However courts have to exercise common sense when applying the law. I paid 2p for a carrier bag in a charity shop yesterday. I could hardly demand a refund if it tore after several weeks of daily use!
As I've indicated, only courts can make firm rulings on such matters but, as a guide, I'd summarise the foregoing as follows:
If the bag starts to fall apart within a month, you've probably got the right to a full refund.
If problems occur within the following 5 months, you've probably got the right to receive an exchange (since it's unlikely that a repair would be practical). The store could, of course, offer you a partial refund (or a credit note) instead but there would be no obligation upon them to do so (or upon you to accept such an offer).
Beyond 6 months you're on very sticky ground. You might still have the right to an exchange but, if the store decided to fight, proving it could be extremely difficult.
Chris