Just managed to get an extra 200 mins on my £15 sim only o2 package. You may be able to add this via the website for free !!
Sign into your online o2 account and on the dashboard screen the "my bill" pages have s been updated and now allow you to view inclusive bolt ons which are included (free) as part of your package.
I have the £15 sim only deal with (unknown to me) 2 inclusive (free!!) boltons : free o2 to o2 calls and o2 web daily (meaning that internet will cost max £1 per day).
There is the option however to switch one of these bolt ons for a range of other options. One of the best ones is 200 extra mins (unlimited internet was also an option). Just swapped my o2 to 02 calls bolt on for this so my sim only £15 a month package now includes :
300 mins (standard)
200 mins (extra free bolt on)
Would never had known about this as o2 never informed me. Worth checking it out if you havent logged in for a while !