I am lead to believe that you can only fit a conventional boiler after
April in exceptional circumstances. This means that they will not be
manufactured in large numbers anymore I presume.

So, if I hired a corgi fitter to install the boiler would he have to
insist I followed Part L etc for the cylinder or would he turn a blind
eye?

I was hoping to convert my system to fully pumped and put in new
controls myself so that when I but the boiler, all the guy has to do is
fit it.