Now fully matched but interested to see what happens if it doesn't get matched
Conceptually, Betfair works by collecting a pot of back bets from anyone that makes them. That pot is then offered out for other people to lay against.
When you specify a stake on Betfair, you are effectively saying:
'I want to lay against X amount of stake/back bet money. If I win the lay, I will claim X amount of money (the stake value). If I lose the lay, I will pay out liability on that X amount of money (stake value X odds).'
Partial matches happen when you place a lay against a stake value, but there haven't been many back bets laid and the total stake pot is not as high as the stake you specify.
If the lay remains unmatched, you will not win the stake you wanted, only the max stake total that was bet. You will only pay liability on the max stake value put into the pot. This is bad news as the point of matched betting is to counterbalance the free back bet you've made elsewhere, and any variations to your lay bet compromise the mantra of 'equal profits for either outcome'.
Always check the £value at the far right of Oddschecker. This tells you how much the back bet stake pot is worth. You can then easily see if there is enough there for you to lay your required amount against.
Remember: you are laying against other peoples' back bets. A big footie match will attract plenty of back bets so you're unlikely to be unmatched in the first place, or remain that way for long. A very odd event - over 6 goals in a women's lacrosse match in Cuba - is unlikely to attract many back bets so the amounts you can safely lay against are limited.
Use your heads. Don't make unmatched lays.