Mark and I went to Staples in hope of finding some Sharpie pens (we should be so lucky!!!) but what we did find was some perfectly sized bags for trading cards in toploaders.
In the States they call them "team bags" - what you need in the UK are "Flexocare Polythene bags" Remember your card should be in a penny sleeve and toploader before putting them in these. But they are perfectly sized (89mm by 114mm is what you want - they also say 3.5 x 4.5 inches on them if you want to double check

).
Woo hoo!
So all buyers from "MakTheYak" will finally have everything properly packaged with a team bag and everything*, which is good news as I am likely to have some SS spares - prior to this I used to hold the end of the toploader shut and sellotape it together. I held it shut at the end so it didn't risk getting the card stuck to the tape.

*shameless self promotion alert!!!