Appreciate what you say DT, yes, the price someone bid can drag execution prices down, this bid price then disappears from the record. But doesn't that just lead back to the original question? - was the trade based on genuine prices offered or was it a case of spoofing?
Obviously, this isn't the CME, so different rules seem to be there.
What the punter needs to know here is quite simple - why did the stop get executed - the broker should be able to point to published rules. It'll either be....
1 - the exchange rules caused the stop
2 - some sort of process at the broker fired the stop based on the bid going way below the stop price - almost a margin call type thing. At that point, it could be that the best Bid was on NEX and it got executed there
First thing to do in this situation is to simply ask why the stop was executed. Based on what rules.
The chances the broker stole his money are close to zero. As has been pointed out - it makes no sense - the risk far outweighs the rewards.
The OP is talking about multiple parties colluding to defraud him of 700 quid. This is a lot less likely than him not being aware of the rules. For sure, he's made 1 rookie mistake of thinking historical executions on a chart represent "price" when the prices available at any one time are NOT on the chart.