I have limited sympathy:
"Didn't even get to put offer on as we were told they had been offered the asking price." OP had ample prior opportunity to offer the asking price;
"We were never even phoned to be given a chance to outbid it. Left a considerably lower offer on the table and asked to be kept posted if it went sale agreed/deal fell through etc." Surely, when OP left the 'lower offer' on the table, the opportunity was there to instead leave a higher offer on the table, if that was what was wanted.
If you put an offer on a property and subsequently discover that it went sale agreed with somebody else at a lower price, you might justifiably feel aggrieved. But if a property is sold and you merely might have offered more for it, I see no cause to blame the estate agent. A prospective buyer who wants to be sure of securing a property should at a minimum ensure that he\she is the current highest bidder. If you suspect that there is a 'phantom' bidder against you, you may decide to step out of the bidding, but if it turns out that there actually is another bidder, then that is really just the luck of the draw.