Guarantees are completely separate to your statutory rights
A guarantee is a 'gift' which some manufacturers (or occasionally retailers) give away with their product. The manufacturer is free to determine the duration and conditions of the guarantee. If your guarantee expired after 6 months, then you've got no right to claim under the terms of the guarantee.
However, your statutory rights are related to your contract of purchase with the
retailer, not the manufacturer. Under those rights, you can complain about a faulty item for up to
6 years from the date of purchase.
The
retailer is responsible for remedying any defect, which occurs during the 6 year period, due to an
'inherent fault' with the product. (e.g. poor quality components or poor manufacturing standards). During the first 6 months of the 6 year period, there's an automatic assumption (unless there's clear evidence to the contrary) that any defect that occurs is due to an inherent fault. Thereafter, it's up to the customer to show, based upon the 'balance of probabilities', that an inherent fault is the most likely cause of the problem.
You should return the buggy to the retailer and demand that they remedy the fault. (The retailer is obliged to either repair or replace the buggy. You can indicate which of these two options is preferable but the retailer is allowed to reject your choice if it's not economically viable. You have no right to demand a cash refund). When speaking to the retailer, you should make it clear that the fault is obviously due to an inherent fault and refer to your rights under the Sale of Goods Act 1979.
http://www.moneysavingexpert.com/shopping/cons umer-rights-refunds-exchan
Chris