Hi all, thanks very much for the responses thus far, it’s much appreciated. That’s a really handy bit of info regarding a Lawful Development Certificate and I’ll definitely explore this route.
As regards complying with the rules, I believe I have albeit there does seem to be a caveat regarding privacy which is
“The decking must not affect the ability of neighbours to enjoy their own gardens, or risk any intrusion to their privacy – for example, by being high enough to see over their fences”
As it is a sloped garden, if I was that way inclined I could stand at the edge of the decking and look over their fences. This is why I mention erecting privacy screening at the far end of the deck to effectively counteract this argument and, provided this screening isn’t taller than 2m (is this measured from ground level?) I can’t see I’d need permission from the neighbours despite their protests.