fullfact.org/europe/our-eu-membership-fee-55-million/
"So overall we paid in £8.5 billion more than we got back, or £23 million a day.
The Treasury figures note payments the EU makes directly to the private sector, such as research grants. In 2013, these were worth an estimated £1.4 billion, so including them could reduce our net contribution further still."
So all of the payments made to the UK would work out at 7.1bn per year, about £136 million a week net payment.
So the £350 million figure is utterly wrong, further to this a 0.5% decrease in growth wipes this net contribution from the tax take entirely.
Using cost benefit analysis the access to the free market is worth far more to the economy that this figure in terms of GDP and generated tax take ( EU migrants in their first year pay in 2.5bn more than they get in benefits for example). EU membership comes out as a significant net present benefit figure.
Further up thread there was a debate about the semantics of what would happen to the money, had it actually existed. "We send £350 million a week to Brussles, lets fund our NHS instead" is pretty explicit about where the money was going, you can try to say that it was a we could fund our NHS claim, but it wasn't.
It was brilliant post truthism, a falsely large figure and an appeal to emotion.