Depends on what you think it is worth - have a look around the neighborhood and see what other similar houses have sold for. Find out what their situation is (if they have a house they want / put an offer in on they are far more likely to accept a lower offer).
Find out how long they have been trying to sell for.
Do a search on somewhere like www.nethouseprice.com and if they purchased it in in the last 5 yeears you can see what they paid for it.
On one house I purchased, I offered the full asking price as I believe it was a bargain - on an other house I viewed, it was up for �140,000 and I offered �90,000 (as imho that was all it was worth).
Dependning on the above, I would make an offer between �155,000 and �180,000.
The estate agent is legally bound to give the offer to the sller, so even if they fob you off with "no its too low", ask them to submit it anyway - if you don't ask you don't get.