Moral of this story:
        Don't rely on PIC protection diodes and a series
        resistor for input signals which swing substantially
        outside the PIC's power rails.

I implemented a "cheap and nasty" PIC RS232 receive interface
as proposed by microEngineering labs in their BASIC compiler
manual, using a single 22K resistor from a PC serial TX data
pin to the PIC receive data pin (and a similar arrangement with
a smaller resistor for PIC TX data).  As I had not used their
BASIC compiler before I initially presumed that problems
getting some simple code going was due to my misunderstanding
of the code syntax in the manual (the language is simple but
the manual is utterly diabolical - at least the compiler is
better than its manual!)

But no! - the problem lay in the interface.
Adding 2 Schottky diodes to the PIC pin to clamp voltage
excursions to (almost) ground and 5v supply cured the problem
and RS232 input worked as desired. While a number of people in
this list have proposed using the PIC's protection diodes to
clamp voltage excursions outside the power rails it is clear
that, in this case, this practice caused the chip to
malfunction. This is of course rather poor design practice but
is a known 'trick' in many other circumstances - especially
during fault conditions. I am not certain whether the positive
or negative excursion caused the problem - I'll go back and
check later - at the moment the Schottky solution works OK. Vin
high current is about (12-5)/22k = 0.3ma and Vin low is about
12/22K = 0.54ma. Incidentally - the PIC data sheet have Gnd and
Vpp as maximum Vin limits with no reference, on a quick scan,
of acceptable maximum currents when the protection diodes
conduct.

(The transmit circuit worked with no problems - as would be
expected as the PIC is driving a high impedance. While this may
tend to pull to one or other rail the affect will be swamped by
the PIC's low output impedance.)

In this case, to implement R232 a simple "proper" design could
be implemented, probably, using resistors alone. 2 resistors
and 1 transistor is the standard el-cheapo solution in such
cases (1 R from P.C. to transistor base, 1 R in transistor
collector, (back diode base to ground if you're squeamish). I
will be using an opto-coupler ultimately to protect the PC and
the target object from each other.