Salary overpayment: tax and NI?

PlanHappy

Ok, so there's a bit of a situation developing.

I moved to a new job quite recently. I was due a few days' pay in lieu of holiday, and a few deductions in terms of salary sacrifice benefits that were supposed to be spread out over my salary for a year, and I also left mid-month, so I wasn't sure what my final pay was meant to be. After leaving I emailed the company to find out what they were going to be deducting so that I could plan my outgoings, but they never responded. Then I got paid... and found that they had simply paid me for the whole month as though I were still employed. This was definitely incorrect.

My faith in the competence of the employer's staff is so low that I didn't contact them because I knew that they would all lose their heads and send me letter after letter with ever-changing amounts of what I owed, what deductions should be made, what shouldn't, etc. - and I didn't myself know what the correct figure was. So I put a roughly appropriate amount of money aside, with a bit of a contingency margin, and waited for them to contact me once they had figured it out, rather than throwing them into a panic and resigning myself to spending the next couple of weeks on the phone to them.

Then another month went by and - despite my new employer getting a tax coding notice for me, so presumably HMRC have been advised that I've left - the old employer paid me again, for a full month. By this time I'm just frustrated.

So now I'm weighing up calling them and trying to force them to sort it out, or continuing to wait until they come to me and just whacking the money in a high-interest current account in the meantime. The only thing that concerns me with the latter approach is what happens with tax and NI. When they claim it back, will they ask for the gross amount and leave me to wrestle with HMRC etc. to get a refund (in which case I don't want to compound the problem by letting it go on, but would also consider putting in a complaint)? Or will they ask for the net amount and sort the rest out themselves?

(This is a huge employer - I'm talking global - and they aren't missing the money. I fully intend to pay it back when I have to, but nobody's going hungry while I don't, and I don't really relish giving up my time to explain to them how to do their jobs.)