Does anyone here have experience of transfers in TXL (Berlin)

Specifically I have a flight (OneWorld Partner) from the Middle East to TXL, and then BA TXL to LCY. The former arrives TermA, and as far as I can tell, the BA LCY flights depart from D.

As they are on separate PNR's I guess I have to go landside, but may yet be able to travel HBO.

Any advice most appreciated.

thanks in advance PWOZUK