they are early 30s and you are recommending saving bonds? why?
OP - As someone else has already said the Rebel Finance school on YouTube is a brilliant resource. Everyone's individual circumstances are going to be slightly different but there are some key principles that will generally apply to many of us.
Sounds like you are starting from a decent financial foundation. You have £20k in cash which is a healthy emergency fund, some people may actually say its a little high even, but with children and not knowing what your employment industries are its likely worth holding onto! If you haven't done it already I would get that £20k within an ISA tax wrapper this financial year - actually do it now, there is all kinds of talk about the limit being reduced so get it protected while you can.
Your pension seems decent, sounds like you are taking advantage of your employer match which is great! Your partner's doesn't sound as good, they may want to consider setting up a SIPP along side it as if they are on NEST the charges and fund choices aren't very good at all.
Whether you pay additional on your mortgage or invest in a S&S ISA is a personal choice. There will be a mathematical answer and a psychological answer depending on your mortgage interest rate. Personally, my mortgage rate is 4% at the moment, I'm choosing to not overpay as I am getting slightly more than that on my cash savings and am expecting to get more long term via my S&S ISA investments.
Think about what your financial goals are and that will then better inform what the most appropriate steps are to achieve them. Remember, its not al or nothing, you could choose to take the £1k surplus you have a month and split it between investments and mortgage overpayments.