As a finance broker I can tell you that in this current climate there are deinitely life spans on offer letter. And if it does expire on Tuesday then it is too late already - as there is no way that the mortgage can be drawn down on Tuesday (since I presume that deposits have not been paid etc).
They may have their own timetable - eg the mortgage has to be taken in 2 weeks time, so if you don't agree on Tuesday then there won't be enough time to complete.
With regards whould you take their offer - well you ar getting �50,000 more that the value of the property. I don't know how much it would cost to do up these properties or if they are borrowing that money as well.
If they plan to make �25,000 profit and you are looking at taking away a possible 20%, then they may think again.
If you have no mortgage on the property and don't need the money, then you could hold on - but of course there are not many people who can get mortgages for development at the moment, so teh number of buyers is much smaller than a year ago.