Buffer tank to partially run on Octopus Go (Faster)

[funks81]

We have a multi zone open vented heating system powered by an oil boiler with about 70% UFH downstairs - the rest of the downstairs and the whole of upstairs is radiators.

I am considering fitting a buffer tank (direct unvented cylinder) on the heating return to the boiler, as it does often suffer from short cycling, because at times only 1 small UFH zone is calling for heat.  I am also contemplating using an immersion heater to heat this tank overnight on my cheap Octopus Go Faster tariff.

  

1.  Does this buffer tank require a vent or is it considered to be the same as putting another radiator into the system?

2.  I haven't yet done any calculations for what size of buffer may be required, however, I was thinking of going fairly large - around 300L in order to take advantage of the overnight electricity.  Apart from the heat losses - are there any other downsides to this?

3.  What are the disadvantages to plumbing on the return to the boiler?  I've seen a lot of diagrams comparing two and four pipe configurations - but not simply plumbed into the return.