End of rental condition

I have been very lucky to have had very good tenants for the past 3 years. They have purchased their own home(fantastic news) so given me notice. The flat when they moved in has just been repainted. My question is this- What state should I expect the flat back in? Should they repaint before handing back?

Do NOT ask for a repaint after three years . That is unreasonable. Fair wear and tear is excepted. It’s normal for redecoration to be done every 3-5 years. That is what their rent was for.
It should be returned clean and free of damage in the condition it was given to you fair wear and tear excepted.
However, if they decorated it with your permission and your condition was that it was returned in the colour scheme it was let in, that is a different matter. If they are smokers and the walls are tar stained from the respiration and you have a condition in contract referencing that you may ask for redecoration or covering the cost of redecoration (read what your contract says).
Your contract should have terms of how the property is returned to you and terms and conditions related to their deposit. Send them that section again.
Send them their inventory again so they know what it looked like in the first place.

DPS states it is the Landlord’s responsibility to remind the tenant of the condition you want the property returned in.
Write them an email telling them exactly what you want done skirting, architraves, windows internal and external, mould, oven, bathroom, tiles kitchen, carpet cleaning, white goods, refuse removal etc.
Send them videos off you tube of how to clean at the end of a tenancy.
If they have issues, and want clarification, offer them advice and go in and complete a pre-inspection so nothing is missed.
You may want to offer them a chance to reclean even after they have left and returned keys if it was not done to your satisfaction ( although they may decline).
If you need to make a claim you have done everything in your power to help them gain their deposit back.