To let you all know that Sigma did provide the fix. After sending my lenses via Parcel Force recorded and insured delivery for £28 I received the lenses back a week later for a charge of £5. They now auto focus in live view on the K3 and still work okay with the K5. Big thumbs up for Sigma, big thumbs down for Royal Mail (very expensive).

Recently received this response from Sigma on this issue.
Looks like their after sales service is as we would hope. Big thumbs up for Sigma.
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Sorry for the delay in response. We had to contact Sigma Japan regarding the issue.

They have informed us there is a software update available for each lens.

Please go to http://www.sigma-imaging-uk.com/returns and follow the instructions for returning the lens to us. If you have the original purchase receipts we charge £5 to cover the return postage. If not the charge is £37.99 inc return delivery.

Turn around time is about 2 weeks.

Good idea, but sounding like it could be expensive, will contact Sigma UK first about it.
Odd thing is the lenses will focus just fine in View Finder mode. Curious.

Err, good suggestion but already did. Spent an afternoon going through all the lenses in all combinations of settings. This appears to be a particular K3/Sigma software interface issue to two certain lenses when in Live View. All work okay on a K5.

Would appear to be a huge disincentive to buy Sigma lenses then, even despite their optical quality and pricing. Maybe they don't really value sales to Pentax users enough to be concerned about this problem. I wonder if this issue happens with their C/N/O/S mount versions ?

Any other K3 owners experienced this ?
K3 when in Live View mode will not auto focus a Sigma 24-70mm f2.8 EX DG HSM or a Sigma 70-200 f2.8 EX DG APO HSM Macro II. Hunts 4 times then stops out of focus.
These lenses work just fine in View Finder mode, and all my other AF lenses work properly in both modes.
These two and all the other lenses also work fine on a K5 in either mode.
Think we need a firmware fix for the K3. Issue has been reported.

Pentax K3 + K5 both gripped, Pentax 12-24 f4, 18-55WR, 100 Macro WR, Sigma 8-16, 24-70 f2.8, 70-200 f2.8, 30/1.4, 50-500 + a bunch of nice old manual focus glass.