In the end I couldn't find that any of the links from Powertools allow both fare bucket and HBO to be specified. Trailfinders on the high street wanted a £20 or so fee, so that ruled them out.
A call to BA got the phone fee waived due it not being possible online. I got confused somewhere along the line as to why when there is a £15 oneway fare difference in N for HBO vs standard on BA.com that didn't add up to a £30 discount on the ITA standard fare for /f bc=N. What I expected to be £90 turned to be £99.
Definitely a learning experience! Prob simplest ti just call BA

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