All inclusive utility bills with rent

I rent several student properties in Greater Manchester area, and am considering starting to do "all inclusive" rent (ie: rent will cover all gas, electric, water bills etc). Up until now, have just charged rent and had all bills in tenants' names.

However, seems that increasingly most students want/ prefer/ even expect, to pay higher rent and not have to worry about bills.

I am happy to consider this, but obviously fear they will leave heating on 24/7 (as many of them do! There houses are roasting sometimes!) and the additional charge will not cover it.

Was wondering how much other landlords add on for all inclusive deals, how much you pay annually (roughly on average) and if it's turned into a nightmare. I don't want to charge a sky-high figure but don't want to pay out £1000's in excess bills. Houses are 3 & 4 bed terraces, new-to-5 year old combi boilers GCH, double glazed and attics insulated.

Any thoughts much appriciated, many thanks.