Sorry for the noise, I've solved it, turns out makeup pages still have headers which you need to set to zero if you want uniform margins:

% Title page layout
% \definemakeup defines a layout with the same name, which we alter.
\definemakeup[titlepage]
\setuplayout  [titlepage][width=middle, backspace=20mm,
                          height=middle, topspace=20mm,
                          header=0mm]

did the trick for me.

Incidentally, what threw me off was the phrasing in the manual:

4.19
Makeup
A document may have a titlepage, a colofon and some pages that are not directly related to the
main part of the document. Mostly these pages are not numbered and can do without headers
and footers. Because their layout needs extra attention we prefer the word makeup for defining
their specific layout.

Since my makeup pages indeed don't have page numbers, I presumed they wouldn't have headers either. I may have read too much into that, but perhaps that passage can be improved?

On Fri, Dec 19, 2014 at 3:33 PM, Mark Szepieniec <mszepien@gmail.com> wrote:
I'm having trouble resetting my margins in makeup pages. I want the makeup pages to have equal topspace, bottomspace, backspace and cutspace, regardless of the standard layout of the rest of the document.

Following Wolfgang's recommendation (http://www.ntg.nl/pipermail/ntg-context/2014/079409.html), I tried the following:

\setuppapersize [A4][A4]

% Standard page layout
\setuplayout    [width=middle,  backspace=1.5in, cutspace=1.5in,
                 height=middle, topspace=0.75in, bottomspace=0.75in]

% Title page layout
% \definemakeup defines a layout with the same name, which we alter.
\definemakeup[titlepage]
\setuplayout  [titlepage][width=middle, backspace=20mm, height=middle, topspace=20mm]

\starttext

\startmakeup[titlepage][top=,]
bla
\stopmakeup

But, on my system, this puts the "bla" too far from the top of the page, and it looks as though the topspace has not been set properly. (Backspace seems ok), Am I doing something wrong or is there a better way to achieve this?

Also, the manual says I can use the command \showframe to display frames to help in debugging, but that doesn't seem to do anything for me?

Thanks in advance,

Mark