This is the WEX form I used to use instead of a written reference (my college gave it to me). I used it because I once asked for a written reference from a lambing placement 3 years ago and I'm still waiting for it to arrive in the post
Some tips:
If you're doing a 5 day placement Monday-Friday, ask for the reference on the Wednesday (for example). This way they've had long enough to get to know you but you can still pick it up before you leave
If you can't ask on the Wednesday, forget or have to finish the placement before the end of the week, give them a self addressed and stamped envelope so they can post it to you
If you're still waiting for the reference, just drop an email or give them a ring to remind them- in most cases they've probably just forgot to send it
I found a form like the one I used was more useful because it's much quicker to fill in- vets/farmers etc. Are busy people and appreciate anything that will save them time. As you can see, it tells the date I was there, my general performance and was signed by whoever was in charge. Unis typically want references just to prove that you've done the placement so will accept this as well as a long lamenting essay written about how awesome you are & how you saved a lamb's life when it was trapped in it's amniotic sac etc.
you don't need a reference from every placement you've done- most unis just want proof that you've done the minimum amount of WEX that they require. So if you didn't like a placement or they're being funny about giving you a reference, you can leave it (although this depends on where you apply as I know Liverpool like to have a reference from each type of placement, but I'm not sure of the details)