You only need it when your current tenant moves out, so when / if they give notice, get one done then.
I'm not having one done, as my tenants have been in a year, and are not going anywhere at the moment.
According to my letting agent they are not worth it, as they don't take into account the age of the property or its construction; it is however another hoop we have to jump through to satisfy Brussels.
This makes me laugh, as I live in Brussels in a rented property which is sooo cold and uninsulated it just isn't true, and I am having to buy draught excluders as the posh looking double glazing doesn't fit well. Thank god for the shutters, at least they provide some measure of insulation, but completely block the light. I can either sit here cold without the lights on, or warm, and with the lights switched on.
No, I an't move - the MOD rent the house and we rent from them, so it's this or something worse!