Hi there, how often can the 72hour visa be applied? ie Can you leave China and then return, repeat

There's no limit. You can do this every day if you want to.

Thank you,
So London > Sha (3days) > HK (3days) > Sha(3days) > London,
Therefore using it twice, is fine?

Many thanks,
And with this above scenario, is it Ok to have 2hour transfers Beijing during the trips between London and Shanghai?

Of course it is.

Not sure if you are correct on this one.... You would then pass immigration in Beijng (as Shanghai -Beijing is a domestic flight) on a 144 visa that was issued in Shanghai...

I asked the immigration officer this question just two days ago while leaving Shanghai: can I take a plane out of Shanghai while on a 144 visa?

It is OK to take a local (return) flight to for example Hangzhou or Nanjing (inside the Shanghai, Jiangsu, and Zhejiang provinces) instead of the train, as I did, as you don't pass immigration. When you pass immigration on a 144 visa it has to be for a flight directly out of China, without any transit inside China.

So, shortly, flying out of Shanghai on a flight to a third country with a transit inside China is not allowed.

Hi, thanks. I see what you mean.

How about the other direction-

Lon > Beijing > Shengzhen > HK
72hrs in shengzhen, airport transfer in Beijing, boat or flight to HK

That is totally not going to work. You have two stops in China and moreover you can't transit in Shenzhen unless you have a visa.

My bad, didn't see your Beijing layover. That would not work if you have more than one stop in China.