ABORT, ABORT!

Having not done any cross-country flying for some time I thought it was about time I did something more than just a local flight and a few circuits here at Hamilton or flying the Stearman when we are over "at the beach". And when my rather-nervous-flyer wife said she would like to come on a flight over to Tauranga for an expensive coffee while I went up in the Stearman it seemed too good an opportunity to miss.
So, having booked Archer III, WIT for this morning we trekked over to the club for 0830 to preflight and get going.  I had been somewhat nervously watching the weather over the past few days but it was clear at Hamilton airport, the bad weather at Tauranga was not forecast to arrive until 1400 and, although there seemed to be quite a lot of low cloud around between us and the Kaimai Range, I knew from past experience it was only only really possible to assess whether the range was passable once up in the air (I have taken four different routes over the range or through the Waihi Gap in the past).
So, we taxied out to the run-up area beside the threshold of 18R and the first trouble struck.  On engine run-up I switched the left magneto off; all good, 50 rpm drop at the worst on the right mag, and then tried the left mag alone; oh ****! 300 rpm drop at least!!  Having encountered this situation before I reckoned it was likely fouled spark plug(s) on the left mag circuit so I leaned off the mixture a couple of times while revving up and tried again. No joy, still at least 200 drop so I tried leaning off one more time and, hey, all good, only a 50ish rpm drop now and it was likely fouled plug(s) after all.
After that we took off from 18R without any dramas and headed off on an East departure via Scotsman Valley (EastNE).  Once clear of the zone at around 2000' it was obvious there was a problem.  A sea of cloud confronted me more or less all the way from just beyond the valley all the way to the Kaimais with only the top of Mount Aroha visible poking out above the clouds.
It wasn't all that great where I was, either.  I had been following a Katana about 3-4 miles ahead and spotted him turning around and heading back towards Hamilton.  I also had needed to descend to around 1200' to avoid cloud.  It looked a little clearer around Morrinsville to the North so I did a climbing turn to the left to head that way and avoid the oncoming traffic.
Once a little to the South of Morrinsville I had a good look around.  The visibility looked very poor to the North as well as to the East and tracking North to go through the Waihi Gap did not look an attractive option and I made the decision to return to Hamilton (my good lady in the right seat was looking a tad nervous at this point).
By the time I got my radio call in to the tower I was skirting the Northern edge of the control zone and was cleared to join straight in for 18L (no IFR or commercial flights in the vicinity, then).  An uneventful approach and landed just after a Katana which I think was the one that passed me in Scotsman Valley.  A good call in the end as I later found out from Pete, the Stearman instructor, that the weather "packed in" at Tauranga at lunchtime and it may have been a little dodgy or even impossible to get back home if we had got there.