Hi Pebbles,
Take some time to do some research into other properties in the area, their selling prices and features and then adjust your offer(s) accordingly. You mention that the 2 extra 'bedrooms' aren't within building code/regulations - be wary of that, has the work been done to a satisfactory and safe level? Also, according to the local council how is the property listed? do they see it as a '2bedroom house'? or a '2bed house with loft conversion'? go by what the rule book says not what the estate agent's selling/marketing spin is.
With the way the market is right now, there's no harm in offering 10% less and then upping your offer if a counter offer comes in, I'd still say be prepared to go up to full asking price if needs be and of course taking into consideration the above points too (if all work is good, it is or can be classed as a 4bed etc etc)
Good luck!
PS last time I was in cov was 2 weeks ago, just by Stoneleigh Park where I was an exhibitor at a trade show, had a fantastic meal in central cov somewhere and got shouted at for not visiting the family members there - go move just outside of cov that kinda way?! the house I remember seein were quite nice!
Dennis! West London & Slough UK!