What does the £400 per month cover, is that literally just rent/mortgage and utilities or does it also cover things like TV license, insurances etc? Do you have a fund for things like Christmas, birthdays etc or do you just spend on these as and when? How much do you and child need for things like clothes, haircuts, beauty treatments? Do you drive or have other transport costs? Basically unless the £465 is literally all your outgoings and the remainder can purely be spent on savings (in which case well done!) I would do a full financial audit on yourself to find out how much you actually need to spend on 'living' and how much can be directed to savings - MoneySavingExpert has a really good tool for this.
I roughly use the 'thirds' method i.e. one third of income on essential costs ie housing, bills, commute costs, one third on discretionary spending (into which I include clubs and activities, subscriptions, clothes, Christmas/birthdays, holidays - the latter I set myself an annual budget and put away money for every month) and one third on savings, including pensions. This can give you a rough guide of how much to save although if you have very high housing costs or a very low income it doesn't always work, but (depending on what that £400 actually covers) I would have thought you could look towards saving £500-£600 pcm.
In priority order I would:
-Ensure you have 3-6 months expenses somewhere readily available
-Ensure you are making maximum contributions to your pension (most tax effficient)
-Put anything left over into a stocks and shares ISA, premium bonds and/or a index linked fund OR into one of the savings vehicles for DC, depending on your risk appetite and what you want the money for, if e.g. you need a new car or to do work on the house then this may need to be more accessible that if it's for DC or longer term it can be less accessible.