Do you actually want to continue being a landlord, or do you want to sell and invest the money elsewhere?
If you want to continue being a landlord, I wouldn't chase the top of the market. The two biggest financial risks as a landlord are of the tenant trashing the house or withholding rent and needing eviction. So I would value tenants with a reliable income who look after the property.
Is their income reliable e.g. a pension? Or are they relying on benefits which may or may not be reliable? Even lovely people would stay and wait to be evicted if they become unable to pay their rent and are reliant on the council housing them.
Setting your price, I'd remember to include not just the mortgage payments, but also to build up a contingency fund for repairs and if they do stop paying rent.
Also a word of caution that getting an undisclosed dog I'd regard as a bit of a red flag. Yes people do get undisclosed pets, and I agree that pet owners can be good tenants, but an undisclosed dog is a bigger step than say a cat.
Also a bit of a red flag thinking they will stay in the house forever. I've never been a landlord myself, but amongst friends and family I've known people who have seemed very nice tenants.... until they are asked to move out. As in their mind the house was theirs for as long as they wanted it and they have reacted very very badly, and refused to move. I've also known cases where these tenants have done unauthorised "improvements" or changes on the house, as they regarded it as theirs.
Ultimately I suppose it depends if you want the role of being a landlord, which comes with emotions and dealing with people's homes and the hassle of repairs and ultimately can mean being hard hearted, or would rather sell up and invest in the stock market or similar.
If you do want to continue being a landlord I'd increase 8-10% and discuss you may increase further in future years.
Best in mind the EPC requirements may increase to a minimum of C soon which will cost you money if property below this. And that in general government is moving in direction of making being a small private landlord less lucrative. So you may end up needing to increase rent in future.
Also thinking ahead with their age I'd bear in mind they may want to make adjustments to the property in future, they may not be able to manage the garden one day, in future there may be only one of them living there with less income. It sounds hard hearted, but that's why I've never become a landlord, as I haven't fancied making money out of people's homes and needing to think this way!