Then would there be an option to add BT Sport on the odd month I would use it then cancel for the other months. For example, I would probably not use BT sport between May and October
Yes, you can take BT Sport for 30 days. Call and immediately cancel.
In my case when I called (I have Player (M) TV), they made me an offer to take Full house TV (XL) for £6/month for 6 months. So I took that up rather than pay £18/month for a couple of months. All convieniently ends in March after the last 16 of the Champions League, at which point I am unlikely to watch BT Sport again until August.