I think you want to do high-side switching with N-channel Mosfets.
BUK100-50GL is a N-channel type.  You can change to use a P-channel,
or
switch the load on the low-side if it is acceptable in your design.
You are not biasing VGS right from what you wrote.

High-side:

 VCC ---MOSFET Switch---Load

Low-side:

 VCC ---Load---MOSFET switch---Gnd

See the philips web-site datasheet on this device.

If you needs some App. note on MOSFET, I can email you from what I got
from Temic web-site.

Ling SM
i-Pal Communications

>I've experimented with a 0k, 10k and 100k resistor connected from
source to
i/p, +5V on the source (source voltage) and drain going to target Vcc
control.

I find I get 4.5V out when the i/p is floating or tied to Gnd or tied
to
+5V, in all the above except with the 10k resistor where the output
fell to
about 4.1V.

Anyone mind telling me what I'm doing "wrong"?!