What have other houses in the immediate area gone for recently? If possible check the ones without conversions, as you can cite that as a rough floor price.
What do you need to move? If you have no offers in then I would caution against a large drop just to get a buyer: it's possible they will get the survey and then drop their offer further. It's v unlikely once an offer is accepted you'd be able to get any more out of them, so think carefully. When you've been looking at houses, what bracket are they in what mortgage can you stretch to, add this plus the fees/stamp duty (think 6k plus stamp duty for most properties) work out how much you NEED to grt stuff in your price bracket. If you're within a few grand then maybe going from 145 to 140 is worth it....but if you don't have an offer in on a specific property it's probably not the time to fold.
Also, when people say "that's all I'll give" they are SO trying it on. Just say "that's nice but we need xx" (and make that a few grand higher than you want). So if they offer 140, you know they won't pay the 150 asking, so say "ooh it's got to be 148 or we can't trade up", cite you have interest on another house (they don't have to know the stage of your offer). If you like that other house, offer on it and see what they say, let em know you have an offer on the table. If they accept yOur offer, do the sums and go back to your buyer.
Tell em final need is for £145, take it or leave it cos you can't move on your vendor with less. You may well get that 145. If they walk, wave politely and say bye, they may be back next week and offer it, in the meantime keep showing and ask the EA to mention you've had an offer recently!!....
Tho I hasten to add I have only my own experience in this and I quite enjoy a bit of a haggle the main thing is don't be intimidated into accepting a low price just to get a buyer if you don't have a place in mind, when its a rush your price can then be dropped cos it balances the ££ against the need to move fast. But it won't work if the offer isn't enough for you to move on yourself!