Two (non-overlapping) UK tourist Visas. Both valid?

[flecky85]

My girlfriend has a 6 month UK tourist visa which is valid until 3 April 2019.
She applied for a new one starting 10 April 2019 and received it yesterday.

She now wants to travel to the UK from 7-28 March which would be allowed on her first visa but I'm not sure if it is still valid to travel on.

Struggling to get conclusive advice. Some are saying its valid and others that it isn't. No straight answer from UK Immigration either or who is best to get in touch with for a conclusive answer.

Any advice would be GREATLY appreciated.