I didn't think about booking over the phone as I assumed there was an additional fee doing it that way. In hindsight a bit stupid but too reliant on technology!
By doing a multi-leg trip I can do
AMS > MLE outbound
Then inbound
MLE> LGW
LGW > AMS (next day)
but I assume this is probably costing me more than a single ticket of AMS > MLE.
As a side note, if you are unable to make a connecting flight that day, will BA never sell you that ticket?