Yeah - I realised!

I think the problem might be what you pointed up - the minimum amount of Avios. If I try to book LHR - DOH return for 1 person I get presented with the options. I think I might not have enough Avios to pay 50% of the trip for the whole family. Odd I could get it via the app though.

EDIT: It's the multicity. If I book a return flight LHR-CAN I can part pay with Avios. If booking multicity, I lose the option. I guess they need to re-do that part of the website!