I usually carry toiletries( small sizes), a change of underwear, book, mp3player, sunglasses, a few sweets, medicines (be very careful going through Dubai, there was a topic a few weeks ago about what's allowed). Is Manchester one of the airports that is now allowing a handbag as well as a small suitcase as hand luggage? if so you have got a bit more space. Put a packet of wet wipes in with your toiletries, I find them very useful on long haul flights also I have a small travel toothbrush and toothpaste set that I like to carry with me.
I assume you will be using the case that you are carrying your lap top in as your hand luggage case, if there is any space left after everything else, you might want to fit in a change of outfit just in case your hold luggage doesn't arrive the same time as you.

Thanks Judith:

Yeah, the suitcase is a radley laptop case, but there's plenty of room for a change of clothes (I've checked it's within size limits.) I'm packing my hand luggage now to avoid doing my uni work

Not too worried about my main case going missing, we're staying with my cousin in Perth so I can raid her wardrobe if needs be