So posting in follow-up just in case it may help any other newbies:
Waders was spot on. The boom is slightly bent from me having fallen on it.
Solution - When I set the boom, I secure the boom clips with zip-ties (& clip of the extra length, then wrap with a bit of duct tape). Clips stay in place and its all good when sailing. ;-) (A bit of work if I have to change sail size, but at least the boom is still giving good use!)