Mr Twiddy had been tipped to play a leading role at the North-East Lep as it moved into the crucial phase of delivering a plan that aims to create thousands of jobs, and secure a share of the Government's £2bn local growth fund.
Mr Twiddy's two year secondment from his role as deputy director at the Treasury was due to end on May 26.
Nevertheless, his decision, and that of his deputy Sophie Haagensen to help set up Atom, shocked the regional business community and robs the Lep of some talented people.
EVEN if you had stayed is there a risk that Leps don't have the manpower to deliver?
"There is quite a lot in that.
"At its peak One North East had 420 people - the core team here consists six people.
"We don't have the same wide ranging remit, but it's worth noting that we have the same amount of European funding, in fact a bit more, and twice the amount of capital funding to be allocated than the RDA ever had.
"In terms of cash management and trying to create a vision and the right choices we have done a lot with very little.
"There has been huge commitment from the business members. In theory they signed up for one day a month but some of them are doing three day a week.
"I have seen us though the formative period and the working up of the strategy, but you cannot do execution and delivery with the resources that are currently available to the Lep.
"I have six big programmes, most of them are £100m plus. With a six person team that simply does not work. It is something future governments have to address.
"The board and Paul (Woolston) as chair is taking the opportunity to reshape the resource that we have go and to make sure everybody understands that their roles are now about execution and delivery.
"That will mean the nature of the people has to change."
HOW difficult has it been to get the conflicting interest groups in the Lep to agree a shared vision?
"NAME any place and there will be emnities and points of difference.
"One of the most important achievements has been when we have had to get around a table and make collective decisions.
"People had to build relationships and find mutual respect and direction, such as on the recently publish the Assisted Area Map (which includes those areas that will be given priority when Whitehall dishes out regional aid).
"It wasn't an easy process. There were people who wanted it to represent their areas and wards, while those from the business community insisted it sat where business were located.
"From that debate we now have a map that includes Glaxosmithkline in Barnard Castle for the first time ever in its life in the North-East, and it stretches to the area around Newcastle Airport to offer an incentive for development on the south side of the site.
"It was a painful process. But it was a good example of how we worked to get to get a good result for North-East businesses."
WHY leave now?
"There is never a good time to go, and it is hard right now as you are actually seeing things happen.
"If you drive up the A1 near Washington Services you can see a big sign in an empty field that says: 'SME units coming here.' That is because we put money into that project.
"Equally, if you come into Newcastle by train and see the revitalised Stephenson Quarter emerging out of the ground - that only happened because the Lep put money into that project.
"In my career I have seen bits of papers and ideas being passed back and forth, or heard politicians make announcements which rely on your analysis, but it doesn't have the physical manifestation of real things happening right under you nose."
DID you have the chance to stay on at the Lep in a senior role?
"I am sure that opportunity would have arisen. But the opportunity to be involved in a brand new bank based here in the North-East and make that real, create maybe 400 jobs over the next few years, was too good a change to miss.
"As much as anything else I have always wanted to challenge myself.
"Giving up my pension and career in the civil service and joining what is a fledgling company is a new challenge and something that I really wanted to be a part of."
SO, Atom must be paying you a fortune to move?
"No, they are not (laughs).
"I had a quite along conversation with my wife who is used to my foibles. It was about following my heart. The values that Atom is going to representatives was hugely appealling.
"It is a partnership, so it is not offering cash bonuses to its senior team. It looks more like John Lewis than a typical bank. It is not there to make a few people in the senior team rich.
"It has a low cost base because it won't have expensive real estate such as a branch network, or legacy IT system that is falling over as is the case with many banks, nor is it involved in some miss-selling mishap, or has an umbilical cord to an investment bank.
"It will operate nationally on the retail side and regionally on the commercial side.
"So, we will be dealing with deposit accounts in London, mortgages in Cornwall and ISAs in Dundee, and invest that money in North-East businesses. What's not to like?
"To be part of that and create something of real substance was too big a thing to resist."