I enjoyed it, but I'm afraid that Geraldine McEwan is more Margaret Rutherford than Joan Hickson. And though Joanna Lumley was good herself as Mrs Bantry, this did push June Whitfield (not immune from donning the hatpin herself, on radio I believe) as Gran from Ab Fab into my mind rather too much. And I don't think Agatha Christie had the sapphic ending in mind when she wrote it, either.
It rather looks like ITV has managed to get all the luvvies walk-on parts in the series, so it should be passable. I suspect that, as they're close to completing the Poirot canon, they're trying to wean us onto something else.