> I need to check that a value, which will be a slightly variable code value,
> is between an upper and a lower threshold (or not).

Here's an excerpt from a "comparison" tip page I'm working on:

Testing that a value lies within a range

In this example, the value in the W register will be tested to determine
whether it lies within the range specified by the constants loval and hival
(inclusive).  In other words, we're testing loval <= W <= hival.

It might seem that this would take six instructions (using the methods
previously described), but it can actually be done in four:

        addlw   256-hival
        addlw   (hival-loval)+1
        btfsc   status,cf
        goto    inrange

Of course, you can change the last two lines to:

        btfss   status,cf
        goto    outofrange

Cheers,
Eric
http://www.spies.com/~eric/pic/