Thank you all for your very helpful replies.
The barn has not yet been converted, though Class Q has been approved.
The solar panel tiles, whilst a great option, is too different from the existing roofing I think, but as advised by IanR, I would need to re-submit the Class Q notice or go for a full planning application, which I am reluctant to do due to the time constriction.
I think we will stick with the Class Q plans and put in a planning application if necessary following completion.
I'm getting the vibe that the less 'boat rocking' at this stage, the better.
Thanks