Draw up a budget spreadsheet, include everything, salary (after tax and pension), child benefit, debt payments, all bills, commitments (gym / sky whatever). Don't forget cash flow (i.e. when money comes in and goes out). Break all things down into annual / monthly / weekly equivalents to have a comparison idea. There will be arguments about what should be on the list, but at least it is factual.
Stand back and hate how much some stuff costs you!
Set up things to be clear and fair. Salary into your account (contentious - but if you have a spender as a DP then look after yourself and the essentials).
Then either run all the essentials out of your account (mortgage, council tax, utilities, car etc). OR set up a standing order ('pay') into the joint account with the essentials coming out ASAP after the payment has gone in.
Note - Watch out for end of month bank holidays where days can slip - allow 4 days for payment in before arranging payments out - this applies to salary in and payment across, and direct debits out.
Set up a separate saving account (joint or sole - ownership is fairly irrelevant if married, access to it is the issue) to put monthly amounts across to cover large annual bills. Handy if with the same bank so transfers are instant to the paying account.
Perhaps set up another account for actual savings. If a sole account then get printed statements so both can see the contents.
Have a couple of non essentials, or predicable bills in DP's name to keep the credit rating up (telephone, water perhaps). Ensure that they get paid out of the joint account though so it is clear that they are actually being paid.
Now to day to day spending. If your partner uses the debit card then there is a pretty good idea about what the spending is going on from the statement.
Watch out for cash withdrawls, and transfers to personal / friends accounts (agree a personal spend that is appropriate though and set up a standing order to partners account for this)
Review the original budget after a couple of months. Point out that the grocery shop was twice the original estimate. Have a bit of a row, and decide that the Tesco/sainsburies/waitrose local/little/metro is the devils spawn.
Marvel at the stuff that wasn't included. (parking, birthday cards and presents, TV licence). Try to work on those to get the cost down, or your acceptance of the real world costs up. Adjust budget as necessary.
Ask your DP to do the annual money supermarket dance for insurance and utilities, perhaps even the mortgage. Beware complete delegation if they are not particularly good with money though as there are gotchas via those sites.
Some will say this is all control freaky. I believe joint spend should be clear and open to both, and both should work at minimizing costs. If one partner is poor at controlling money then the wellbeing of the family needs to be protected by the other. Unfortunately they could go off and rack up all sorts of debt via credit cards etc under their sole name which married couples are jointly liable for.