Thank you again everyone for all your replies and suggestions. I have taken everything on board. It's been a real eye opener to see how many other people feel this same kind of loneliness and none of us know who else might be in a similar situation! For all I know, one of the school mums I say hello to in the mornings, she may well be very lonely too.
I am sure the problem lies in the fact that I struggle to make small talk. My mind races a bit when I meet people I don't know well and I internally panic for ideas on chit chat type convo.
Interestingly, when I am with my DH and we are chatting to someone, I feel a touch more confident and I don't panic as much with making conversation flow.
I have talked to DH often about my lack of friendship and he is always encouraging me to get to know people, ask the mums back for coffee, etc, but as I said, he is extremely outgoing and charismatic and doesn't have the same issues of making the leap of conversation turning into plans.
I get on well with people, all of DH friends like me and I think basically, people would not guess by talking to me that I am lonely person.
I don't know if this is a real thing or not, but my sister, along with quite a few other people I have known over the years, have all jokingly told me I have a bad case of Resting Bitch Face 
I never knew what to say to that. I always have that in my mind, so I try to be smiling lots to try and counteract it lol
Anyways thanks again, I think I will take the plunge and invite a school mum for coffee one morning and also, thank you for all the lovely PM's I received, hopefully we can arrange a wee meet up for a few of us ladies in the Glasgow area xx