Be very careful - taking anything from the person, even to relock the car/house door could be considered assult in the Uk and you could be in trouble with the Police - Or he just might thump the stuffing out of you for being "clever".
I'm in the UK, do martial arts and built like the proverbial s***house, but even I take a LOT of precautions, knives and guns are not unknown after all. So what can you do? Well, this is what I do, whether you choose to do the same is up to you of course.
1) Make a quick assessment of the customer, are they nervous or edgy? Remember you could be being conned into letting someone back into their house after a split-up so they can take stuff, wreck the place or whatever. Listen to your instincts and never be afraid to walk away (say you haven't the right tools for THAT lock or something). You don't want to spend time in court over a bitter domestic dispute as you were conned into letting someone back into the house.
2) Say it is "company policy" and that your boss insists on the money upfront for every job. Usually the answer is their wallet, money, cheque book or whatever is on the other side of the door they want opening...
3) If so, take out a clipboard and fill in a sheet in advance (always have a few pre-printed at the back of a pad or something) and make a great show of noting down all the details of the car, including the registration/licence plate of course. Make sure the customer sees this, so they will know you have plenty of details to give to the Police, this should disuade a few more of them from trying to rip you off. If it is a house, then make a big show of writing down the address, customer name etc.
4) Ask for their mobile number and then RING IT there and then infront of them, to make sure they have given you the right number. Again say that it is a "legal" requirement you have to do, to help ensure that they do have a right to request entry into the house/car or whatever and again, make a big show of writing down the number.
5) Make sure the form has the correct date, the agreed amount on it and then get the customer to sign it. DON'T sign the form youself first (if the form expects you to sign it as well). This way the client doesn't know your name if problems do occur.
6) Dont fight! You might get a good kicking of course, you might even get a broken arm (makes picking locks difficult), or you might win - Only for them to then lie like mad to the Police after and have you arrested for whatever their diseased mind can come up with. Even if you can prove you are innocent, can you really afford a day or two in court defending yourself, instead of being out there earning? Even if you do win, you might get a false call-out the following week, only to be ambushed by his bigger mates.
7) At the first sign of trouble, call the Police and let them deal with it. They are licenced to manhandle people, you are not!

Even if you don't get paid, how much have you really lost? Not the full £70 or whatever you were going to charge them, really it's just some of your time and a few quid on diesel. Better than waiting six months for your case to come up in court if you retaliate?
If this sounds like being a wimp to someone, please remember that the law in the UK allows you to ONLY use reasonable force to defend yourself or others, but ONLY until the moment your opponent is considered no longer such a serious threat. If a fight breaks out, whether you win or loose, the other person is likely to complain to the Police and then they of course do have to investigate the claim. I personally know someone who went into his own home when he found it broken into. The burglar was on the premises and a minor scuffle broke out. The home owner was fined more then the burglar was, because the very act of going back into his own house could be considered provocative of a confrontation.
I KNOW IT'S DAFT! The law is trying to change thank goodness, so that it is more balanced, but until it is, it is probably best to call the Police yourself.
Just one point of interest, if you have to open a door in extream circumstances, such as a home reposession, then ring the lock Police station and ask for them to be in attendance. Usually they are fine about it, as they are stopping trouble breaking out after all.
Personally, I rarely get anyone trying to avoid paying - I usually get them trying to negotiate the price down once the door is opened, even though they agreed to it in advance! People will agree to a lot when they are locked out, but once the door is open again, they are usually more concerned about parting with their money.
If they do seem reluctant to part with the money, remind them they have now signed a legal contract and if you don't go back with the full ammount, the boss will sell the bad debt onto a debt collection agency he uses - Sympathesise with them and say no-one wants an aggressive debt collection agency after them. Usually they will see sense.
Hope this might help a little?