hi, you are not too old. i am 34 and in my second year there are lots of different ages in my year group, many over 30. where i am studying they value the life experience that people have. dont forget you have lots of transferable skills for example you must have good communication skills from your sales job and communication skills are valuable in social work.Being a mother also will have given you skills that you can use in social work.
i dont think there are part time courses but there maybe. my course is full time but i am only in university for 2.5 days a week with the other time for studying and reading at home. this is really good as i have a son so it means i am around for him as well as having time for studying.
financially things are okay i am not sure of your circumstances but i have a loan and a grant and a bursary and as students we dont have to pay council tax which is a help.
i hope this helps. i would say go for it if you really want to do it i love it and having a passion for the subject definately helps.