A retailer is expected to repair or replace a defective item (free of charge) for up to 6 years after the sale if the problem is due to an 'inherent fault'. (i.e. if the problem was caused by poor quality parts or by poor manufacturing standards).
For the first 6 months after purchase, there is an automatic assumption (unless there is clear evidence to the contrary) that any defect is due to an inherent fault. Thereafter, the customer must show that (based upon the balance of probabilities) the most likely cause of the problem is an inherent fault.
You've got two problems. Firstly, the foregoing doesn't apply to expendable items. (e.g. you can't buy a car and expect the vendor to keep replacing the wiper blades for 6 years). It might well be argued that the parts which create ignition are not really expected to last 6 years.
Secondly, it could be hard to show that the problem is due to an inherent fault. There are other possible causes, after all this time, why the gas fire might not light. (I had a new gas fire fitted, because I couldn't get a small spare part for the old one. It worked for one day before the cat piddled on it and it's never worked again!).
Apart from your rights relating to defects caused by inherent faults, you've got no claim against the retailer. Thousands of people bought Betamax video recorders and then found that they were useless because the format wasn't supported. Thousands more have bought HD DVD machines, only to find that Blu-ray has won the battle and their machines will be redundant. Others bought Kodak Instant Picture cameras, only to find that they couldn't buy film for them because Polaroid took legal action against Kodak. Not one of those customers had any rights against the retailers. Unfortunately, you're almost certainly in a similar position.
Chris