Be carefull if you don't book first time but get a quote
Being a little careful (or neurotic) I did a quote, then went to double check my details and realised I needed a different time for dropping off. Not seeing an easy way to amend my timings I closed the window and started the quote/booking process again.
Shockingly, the quote had just increased from £28 to £45.
To avoid this ridiculous increase I logged out, redid my quote as a non logged in customer and surprise surprise the quote was back to £28 and remained so when I logged back in to book it!