Short update, I finally got LATAM to confirm their UK address for legal docs is the one listed at Companies House (Unit 20.2...)
Have run out of my infinite patience (!!!) I finally issued a money claim (basically a County Court submission that you can do online with a £35 GBP fee).
To my amazement, 3 days after this submission I got an email from LATAM telling me my refund was agreed and they would issue a refund (I have no idea if this is a coincidence or a consequence of my submission). The email made no reference of my Court action.
Of course...not out of the woods yet:
1) LATAM want to make refund to the original credit card used for payment and I don't have that any more (so I have asked they do a bank transfer)
2) LATAM need to pay the refund +interest and my Court fee (that you get back if you win the case) and not just the plain refund (although that would be a start)
3) Despite the email saying they will refund me the money, they do actually have to do something (something they haven't done since last July 2018)
So, the Court case remains and only when I receive the cash will I drop it.
I encourage anyone having problems getting an "obviously" owed refund to take action sooner rather than later (is really simple in the UK) - however, part of the Court process is to show that you have made efforts to resolve the situation (phone calls, support cases etc)