we're catching the tirano to lugarno bus which get's in to lugarno 18 minutes before the train to milan departs .Is this enough time to make the connection ? How reliable/punctual is the bus service?

Lugano? Locarno?

it depends on witch bus company do you use?

Why not just train to Milan?
Lugano is out of the way.

>>>Why not just train to Milan?<<<

The OP is taking the train on that part of the trip("18 minutes before the train to milan departs" ) . The bus appears to be from Tirano.

I suppose you will have a Swiss Pass. Otherwise it would indeed be easier to leave Tirano at 15.10 by direct Milano bound train (ar 17.40, fare 11 EUR).
The Bernina Express bus arrives usually in time. Otherwise there would be a (slower but much cheaper) direct Milan bound train just 30 min later: Lugano dp 18.28 - Milano C ar 19.50.

BTW: Lugano would be a much nicer place for dinner and overnight stay than Milan.

Thanks for your replies- yes I do have a Swiss pass so am arriving on Bernina Express bus.

thanks for the tips about the trains