Thank you so much to everyone for your brilliant advice/experiences/replies. I've found them so helpful.
SlimJim - You're right about "mothers' instinct", and I'm relieved to hear that GP's respect that. I'm glad to hear your LO has now caught up with peers, despite the delay and the difficult birth. It was a comfort to read someone who had a similar experience of birth complications, but whose LO has come through unscathed (albeit in his own sweet time).
OneWay - Thank you for your wise sentiment- you massively hit the nail on the head when you said that it's important to remember that we're not there to be the Dr's friend, they don't have to like us, it doesn't matter what they think of us! I'm a bugger for worrying what impression I'm making and apologizing for my very existence when seeing the doc. But your words have resonated with me, and I'll use that to bolster my nerve. Thank you. And that website link is great - v informative.
HazyJane - I'm so glad your DS has had the right tests and support to fulfill his potential and is a happy great little boy. You certainly sound like you had a good HV! I'd never really seen a HV (until today), just the extremely young dizzy nursery nurse from the HV team instead, who paid a cursory visit at 10 months, didn't check him over (just asked me some general Q's), said she'd call back at 12 months to check on the weight-bearing, then never did
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daytoday and laura4jasmine - thanks for your words of advice and support re: sometimes needing to be a bit "pushy" to get things investigated, and this not meaning I'm an anxious parent. You're right.
So...an update since this morning... After reading your replies I decided to strike while the iron was hot and called my HV team office. For the first time ever I spoke with a real HV (not a nursery nurse) who said she'd come out today to do a proper check on him and if necessary refer for investigations... HOORAY!!
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I dashed about all morning to get back in time for the lunchtime visit, only to be told as soon as she arrived that she wouldn't be doing any checking of him (the GP has to do this apparently?!?!
) and soft-soaped with the same old patronising bullshit as follows:
HV: "Try not to compare to other babies", "it could be nothing to worry about" blah blah blah.
Me: (inwardly) YES! I KNOW ALL THAT! WHAT IS THE POINT OF YOUR VISIT THEN???!!! YOU SAID YOU WERE GOING TO CHECK HIM OVER THEN DO A REFERRAL. WAS THAT JUST A LIE THEN? 
So, off she goes, me none the wiser, just thoroughly patronised and frustrated, and having now missed the GP opening hours (closed on Thurs arvo). Great.
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Sorry for the rant! I'm all fired up with frustrated dynamism now to get this sorted, and have to wait until tomorrow to book GP appt, by which time I'll be in a confused torpor again.