^
So... 4 months after starting this thread I have finally managed to transfer my miniscule number of useless Iberia Plus points over to BA Avios. The recent replies having prompted my to give it another go.
All working fine for me - finally!
The only thing I have changed is to ensure that my IB details are complete (a contact phone number was missing) - so not sure if that was preventing the transfer, or whether some bugs have been fixed recently in the tool. No HHA considerations in this case. Both accounts had been open a number of years.
My only advice to anyone else struggling (and echoing what has previously been posted here) is to ensure that the recorded contact details (email, addresses, phone numbers etc) are complete and identical in both accounts.