You are a bit stymied in chasing the bidding up yourself because it's a house. If it got knocked down to you, you'd be a bit vulnerable to action.
Me, I be tempted to gee the ring up a bit.I'd wait until they were looking smug and the bidding had slowed to "Any more?" and then be tail end Charlie and bid. If the ring doesn't know me, I make it clear by saying "Yes, sir!" They all look and think "Bloody amateur!" and try to see me off. I then quit near the right price, but a good bit below [see above], and I'm the one feeling smug ! I've done this many times. Also, you often lure another tail -ender into bidding (there are lots of us out there, who take advantage of the existing bidder thinking he's got it and then suddenly regretting how much he has bid in his enthusiasm; he's grateful to escape)
They'll be a profit even at £500k, by the sound of it, and that's what they are in the ring for, so they are unilkely to let it go at a bit below that.
It must be very unusual to get rings in property auctions,whatever the gossip, and I have never known one, even in tiny provincial houses. The market attracts too many public bidders, nowadays, who know the prices, let alone speculators who are outside the ring; for example, if one of those saw it going cheap, he'd stick a bid in whatever else he'd come to buy. Where you get rings is in specialist, narrow, areas such as industrial tools or dolls and toys.
And it may well go for near or above the right price; that happens a lot. You only need two people who are keen and sense goes out of the window.