Rent increases in a HMO

Other

How do landlords manage rent increases in a HMO? Usually the turn over is quite high so in the past I have looked at rents in the local area and put the price up for new tenants coming into the house, according to what the current average rate is in the area. 
 

However, I now have two tenants who have been in the house for 3 years with no rent increase and the rents have gone up at least by 10% compared to what they are paying. Newer tenants are also tending to staying longer as there are little alternative housing options. 
 

I am thinking of putting up rent by 5% every 12 months  as a standard for existing tenants to keep up with the local area and my increased costs. With the tenants still paying the lower rate I am considering a 12% increase. What do other landlords do? Do you always increase rent annually or leave it the same? They are good tenants so I was happy to do so for a while but they could be in the house another 3, 4, 5 years, who knows?! I don't think it is reasonable to stay in the same place with no increase for that long...