let’s start from the beginning

http://www.rednaga.co.uk

mixes https and http
your ajax url is set to be https, so it would ALWAYS fall over as you cannot access scripts crossdomain
(my guess is that your host person added something dealing with that discrepancy in your htaccess file, fixing the symptoms, not the cause – just a guess though)

unrelated, and nothing to do with me, but while i’m at it :
for some unknown reason, you also have the following 2x in your page (in itself , that shouldnt be a problem, but who knows what else is going on)
var cro_query = {"ajaxurl":"https:\/\/www.rednaga.co.uk\/wp-admin\/admin-ajax.php","cro_nonces":"ea20d3befc"};

apart from that:
—–a—–
> Because no server will ever be 100% reliable …
not really true anymore these days. there are plenty of hosts that guarantee 100% uptime (though you pay for it)

—–b—–
i cannot tell you about host or whether they are good or not. google probably can

(very simplistically)

it means that you cannot send wordpress javascript/ajax requests from an http:// page to an https:// page
(and vice versa)
they are different domains

go to
http://www.rednaga.co.uk/order-online/
and (try) to put something in the cart, not only will it most like throw an error, but if you now go to
https://www.rednaga.co.uk/order-online/ (by clicking on your navigation) you will have nothing in the cart (or different things)

in short, http:// and https:// are entirely/totally different sites but your http:// site uses scripts from the https:// site and is trying to submit stuff to that one which you just cannot do

you / your host/administrator/whatever should force to redirect ALL pages to https:// or just don’t mix and match like this….

you are assuming that your site visitors will always come through your homepage and then click on things…

that just isn’t the case . when is the last time you searched for stuff , clicked on a link and went to the homepage as opposed to the page where you actually want to be ? (my guess is: about never)

though – seemingly – google has only indexed your https pages (which is good and will eliminate most of these issues) if ANY other page on the web links to any of the pages on your site without https: it will fall over

for example: just go up a couple of posts and we have a link to your orders page without https.
this is indexed by google.

someone comes here, clicks on link, falls over….

again, it’s not quite as dramatic as searching on google for rednaga.co.uk will mostly result in https links it seems, but you really cannot be certain that’s alwasy the case…

=== end of lesson 🙂 ====

> that is the website falling over yes?

can’t be certain for all cases
but what i AM certain about is that it WILL fall over if your are on http://etc etc /etc