The amount recommended by the cruise line (per passenger) is £2 per day for the cabin stewardess and £2 per day for the dining roon steward. This is, of course, optional and is not, as on other cruise lines, added to your bill. I have sailed with this line many times (mainly on Black Watch)and I have almost always found the staff well worth this sum and more. I am sure you will find this a most enjoyable cruise.
I also cruised on Black Watch recently and can confirm that it's £2 p/day. It's certainly good value and we added more to it.
It's a nice boat with a very nice crew. have a good holiday.