Definitely 10% as others have said and in some places well over that. HOWEVER... you can only get a mortgage on what the place values at in a survey/valuation (which you need for a mortgage). So if you offer 10% over and the place values at that (often surveyors will tweak it a bit so that if you offer a reasonable amount over rather than excessive they will make the valuation match the offer) then you won't have to foot the extra.
Example...
House is offers over £100,000
Place values at £110,000 but you've heard there are higher offers on the table so you offer £120,000...
You've got to foot that additional 10 grand yourself in addition to any deposit. It's a pain and I didn't realise this until we bought our first place last year but thankfully we only went £500 over the valuation!
I don't think things are slowing down in a lot of areas, it's scary. Our flat has jumped in value by at least 10 grand in a year (possibly more if we finish decorating) - our mortgage advisor who has his own estate agent came in to "pass on some papers" (i.e. have a nosy) and gave us a sneaky valuation. There was another house on the market locally in a ridiculously popular area - offers over £280,0000. I was in the office when someone phoned to complain that their offer hadn't been the highest on the closing date (£341,000) but they were informed the highest offer was £380,000
We were quite lucky - try and get the seller to show you round! He ended up taking our offer because we had said we could offer him what he wanted and give him a quick sale. There were other slightly higher offers on the table but both wanted a couple of months to close whereas we could move in right away - the seller wanted a quick shift so it didn't go to a closing date. It doesn't happen a lot but you could be lucky!
Sorry I've rambled but let us know how you get on!
