We have a very similar household income to yours. Unfortunately we aren't entitled to tax credits, housing benefit etc as we are over the threshold (26k for one child) but we manage fine.
A few tips for you - save as much as you can before you go on maternity leave. Tax credits etc are calculated on household income for the whole tax year (April-March) so you won't necessarily be entitled to any help. We weren't even though my take home pay more than halved. Use the online calculators to work it out.
Asda and Aldi nappies are absolutely fine. No need to spend twice as much on Pampers. Same goes for clothes - don't spend loads because baby will grow so fast it's not worth it. Stock up on nappies whenever they are on offer, by the time you run out chances are they will be on offer again.
Do a spreadsheet of all your incomings and outgoings. See where you can cut back and if there are things you can go without. It's better to overestimate how much you pay on things than underestimate. Eg don't put down £50 a week for food if its not realistic because then you will leave yourself short at the end of the month.
Pay all important things as soon after payday as possible so you know exactly how much you have left over. I pay rent, council tax, utilities the day I get paid. It means there's not much left but I know that all the important things are covered.
If you have enough money to save then do a standing order each month to a separate savings account. If it's in your current account you might start dipping into it and before you know it it's gone.
Finally congratulations and enjoy your pregnancy. Try not to worry about it too much, it will all work out.