I've just checked a comparison site and Thomas Cook, closely followed by M&S are showing as the cheapest, unless you have them delivered.
The downside of ordering on-line and having them delivered, apart from ensuring someone is home to receive them, is that your bank may charge fees on debit cards for such a transaction - something to look out for!
I have to organise some myself this week, usually I order from my bank, but I think this time I might shop round the travel agents or see if I can get to M&S, with cash in my hand (to avoid those debit card charges).