I did runner work on a feature film in London last year. I see you're from London so I'll try and make my experience a bit specific to how it was for me there.
I don't think I would've enjoyed it as much if I wasn't friends with one of the lead actors/writers, but it was definitely an experience. Your day will start by going to a greasy cafe and ordering 30 sausage or bacon rolls for the cast and crew. You will get many pissed off looks from the locals because of how the kitchen is backed up with your order. You will deliver them to set and probably get asked to load equipment on your way in. After loading equipment you'll then need to clean away all the breakfast bits and - if you're lucky that day - there will be a bacon roll left. You'll also need to check all the refreshment, if you're running low on tea/coffee/coke then you'll need to scout out the local Tesco or Sainsburys. Knowing where they are in advance is a huge help. I almost cried with happiness when I found out there was a Sainsburys in Kings Cross when production was based there for a few days.
About an hour after that, you'll be asked to scout the local area for somewhere to order lunch. Once finding somewhere appropriate [aka cheap] you take some of their menus and approach everyone in the cast and crew to get their order. On the first day this is great as you'll get to know everyones name. Once the order is in, you once again watch a small food place struggle with 30 orders for an hour. When delivering them to set, they will have almost definitely forgot to make something or changed an order. That means the blame is on you and you have to run off to get the restaurant to fix it.
Once everyone has their food and is eating, you can sit down for the first time that day and maybe chat to some of the guys in production. For me it was fine, as a couple of my friends were lead actors, but it's good to chat with the art department and make up as they're always good guys. It also means that you might get to do some meatier things for them on other days. With Art Department I had to get to Stratford for various different paints and then back before production started one day and then I spent a few hours set building and painting.
You'll sometimes be sent off set to do or collect things from places you've never heard of. Make sure you've got a battery pack for your phone, because if it dies you're up shit creek. I had to swan off to Tottenham Court to Primark to quickly buy costumes before and wander around Camden finding a lens rental shop to pick up a lens. Production didnt tell me that they were a day late picking up the lens in the first place though, so you can get tellings off from people who arnt even involved in the production. You wont be refunded for going around on the tube, but depending on the production you'll get a certain amount of transport costs refunded.
At the end of it, it's a very thankless task. People wont thank you if you're doing what you think is above the call of duty, because to them you're just doing what they expect. But if you get involved in a nice low budget project with good people, then you can have a good time. I was an extra in a few scenes, just because they have to bulk out scenes. I was given some on set training one day from the camera department and was allowed to do clapper on one day too, which was exciting. Even more exciting when you see a quick clip of you on clapper on the DVD's special features.
The best day for me was the last. The director came to me, thanked me for my work over the past few weeks and spoke to me about what I wanted to do in the industry. Since then I've done some camera operating on some projects he's directed. The film industry is all about connections and relationships. Look like you will do anything for anyone in the production and smile.
