When we sold/bought a couple of years ago in Glasgow, estate agents seemed to be deliberately putting the o/o price at a level where the expected selling price would be around 10% over (I was very
about this - why start at a deliberately low point and just waste everyone's time?).
As pp have said, the Home Report value was more important. We got our HRV when selling (surprise, surprise, around 10% over the o/o), but paid less than the HRV when buying. The property we bought had been on the market for a while and had failed to sell in the past (was rented out in between) so they were open to reasonable offers.
Another property we failed to get was priced very low, nearly 20% under the HRV and had been on the market for ages. We put in what we thought was a reasonable offer (and nearly our best price) and were turned down flat. Turned out the vendors were a divorcing couple in serious negative equity - it didn't sell and is currently rented out.
So, if you're the only interested party it's fairly straightforward. Find out what the surveyor thinks it's worth (via Home Report) and use that as a guide, not the o/o price. Take into account what you think it's worth and maybe if it's been hanging around a while consider whether a lower offer might be worth a punt, otherwise you should be aiming around the Home Report.
It's fine to haggle a bit and start low. Our buyers put in a really cheeky first offer but a bit of toing and froing between the solicitors over the next few hours got us the price we wanted. I then did the same when negotiating with our place. Took a couple of hours back and forth to come to a price we agreed.
If there's lots of interest and it goes to a closing date and sealed bids, then obviously all bets are off and it's about how much it's worth to you.
Good luck - maybe we were fortunate but I found the process really simple, albeit nerve shredding...