PayPal are not a credit company, so they're not covered by the legislation which applies to those companies. There is no legal obligation upon them to provide any guarantees to those who use their service. (They're no more obliged to help you than your bank would be if you'd paid by cheque). The fact that they do so is purely a matter of goodwill. As such, it's entirely up to PayPal to determine the terms of their guarantee. If they choose 45 days as the maximum period during which they'll provide assistance, that's up to them. You have no legal right to challenge this.
If you made the payment to PayPal with a credit card (rather than with a debit card or a direct transfer from your bank) you might be able to seek assistance (and a possible refund) from the credit card company. You would definitely have had the right to do so if you'd paid the supplier directly from a credit card. It's less clear whether you're entitled to claim against a credit card company if a payment is made through a third party, such as PayPal.
If you didn't pay via a credit card, the only firm you can pursue is the supplier. Their website should have a postal address on it. (It's a criminal offence for a UK online retailer not to include a postal address on their website. If there's no address, contact Trading Standards). Write a letter which states that, because the supplier has exceeded the stated delivery time, you now regard your contract with them as null and void. Demand repayment of your money. State that, if you don't receive payment within 14 days, you'll pursue the matter through the courts. Send the letter by recorded delivery. Keep a copy of the letter and retain the proof of posting. If you don't get your money back within the stated period, visit your local county court to get advice about starting proceedings against the supplier.
Chris