Bruce McGregor's bar in Inverness has an excellent session on Sunday afternoons between 4.00pm and 7.00pm. Good beer and whisky selection, and the food is very good as well. It's quite a small bar space, and very popular, so get there early for a seat. Mainly instrumental to a high standard, but the odd song would undoubtedly be welcomed.
Speaking from personal experience of a couple of visits last year.
If you are in Edinburgh between 27 - 29 March, the Edinburgh "FluteFling" takes place that weekend including a concert of music played on flutes and whistles by tutors Claire Mann, Tina Jordan Rees, and special guest visiting from Ireland, flute maker, performer, and occasionally singer, "Hammy" Hamilton. Evening of Friday 27th.
Still in Edinburgh, there is a very high quality afternoon session in the Waverley bar on Sunday afternoons. 3pm to 6pm, which although mainly instrumental often has some excellent singers in attendance.
Again, speaking from personal experience from last year.
Hope this helps.