Rebecca,
Remember that the (in your view high) price the agent quoted you was in the context of the agent bidding for your business. What evidence can they show you to reassure you it's reasonable?
All too often sellers get emotionally attached to prices which weren't realistic in the first place. And, of course, subsequent lowering of price sends out a signal that is not helpful from the seller's point of view. If you genuinely think the agent's suggestion is too high but you want to go with them in any event, why not instruct them to start off a few thousand lower? If they were right, the higher price should materialise during the sale campaign.
The rapidly rising market of recent years meant that if the starting price was too rich all you had to do was wait and general price increases would retrospectively justify the initial valuation. That's less likely now.
Don't forget to talk to your solicitor. He/she should be able to hold off closing on your purchase for many weeks in the event your sale stalls, often on seemingly spurious grounds. It's a dirty game but others would do it to you - don't feel too guilty...