My DD is only a week or two younger than yours and only rolls very occasionally at the moment, she gets from her back onto her side easily but can't work out how to get to her tummy, from tummy to back isn't much better. Have you tried rolling her over from her tummy to show her she can do it? Then rolling her half way and get her to do the rest? 18 mins is a long time on her tummy so she must be strong!

My DD cannot sit on her own yet, she does lots of sit ups and can sit assisted but will fall on her face if I let her go, opps...

I'd say your DD is on track don't worry, most other babies in my mothers group arn't rolling yet either....