John Rumm wrote:

Only I was just thinking it may be a easy way to measure an installed
(but as yet unterminated) length of cable using a LAN tester with cable
length measurment capability...

Point 1: you need to specify between which two cores the capacitance is
to be measured and whether the third core is floating, guarded or
paralleled with one of the other two.

Point 2: PVC is a pretty rotten polar dielectric, so don't expect to get
a precision dielectric constant (epsilon_r) value from anywhere.
http://www.kayelaby.npl.co.uk/genera...2_6/2_6_5.html
suggests values in the range 2.8 - 3.2 for uPVC depending on frequency,
but presence of plasticisers and fillers will doubtless modify this.

An empirical approach with an LCR meter or bridge is probably the best
way forward.

--
Andy