Should they be calling me in for an interview if they are truly interested?

I've been cold calling and specutively sending in my resume to small boutique firms.

For most jobs this is a very, very low yield strategy, it is much more effective to spend the time finding actual open positions and applying for them. As I mentioned in my answer to your question here I've never seen it directly work for a "professional" role. If there is literally no positions coming up at all through the job boards and recruiters have got nothing then it's better than doing nothing I suppose but I certainly wouldn't be treating it as plan A.

I've had some positive responses - in that Head responds back and says "we do not have anything suitable at the current time. I thank you for your intere>st. We will be in touch if something suitable materialises" blah, blah

Now whilst many would think of this as a blow off, I thought of it as a not now, maybe in the future, and am hoping to email them every 3-4 months with updates. Am I wasting my time?

Sometimes yes it's a polite blow off, but othertimes it is exactly what it says on the tin. I have to say though that following up every 3 months is excessive and far more likely to sour a contact on you rather than help. 6 months is the minimum time I'd leave between contacts and even then only if you actually have something new and worthwhile to add such as new experience or qualifications.

Secondly, my friend said if they were truly interested in me they would call me in for an interview even if there were no positions available, just to see me and have a chat; is this true?

There's no definitive answer to this but your friend has a point in one important respect - if they were interested in hiring you in the near future (say < 3 months) then I'd probably expect a proactive response from them such as a meeting or phone conversation. The absence of such doesn't necessarily mean that they wouldn't hire you though - it's far more likely that they just don't have anything for you. When I've been in roles where I've been responsible for hiring staff it wouldn't have mattered how good a candidate's CV was that came across my desk. If I don't have a role for them to fill I wouldn't do any more than send the sort of response you've got as otherwise I'd be just wasting everyone's time talking about a job that didn't exist.

As a newbie to the job search world, can I just say feelings are overrated and in these circumstances a blunt yes or no from firms would have been appreciated!

While I sympathize you don't know that they are just sparing your feelings, "not now but maybe" is a perfectly valid answer.