Update! My previous reply was erroneous I think! I discovered that, by coincidence, someone had woken the mac up at home, just before I tried it In Tesco! However, I now have it sussed. Firstly I hadn't realised that a wifi setup doesn't have the same MAC address and IP address as an Ethernet setup. So I deleted the wi fi set up altogether to begin with, and set up an Ethernet connection. I found the MAC address, and Ip address of that -System preferences /network/Ethernet (that's what I called my Ethernet setup) advanced and the MAC address is in the hardware tab.
Having got those sorted, it works fine now every time. I've even put the wi-fi back on again. I've downloaded PocketCloud and logmein apps to my iPad. I like them both actually - can't decide which I prefer, but they are both great and easy to use.
I just thought this might help anyone else as Mac illiterate as I am, as it took a lot of trying to get mwol to work! But I finally got there!