We ended up going with Vinetrekker who I would heartily recommend. Paul, the guy who runs it, was great company and very knowledgeable.
We visited:
Coldstream Hills - bumped into James Halliday in the carpark who was hosting a private tasting for Eddy McGuire up at the farm. Very friendly guy. Wines were enjoyable but the pours were too small to get much of a sense of them I thought. Highlights were the Deer Park Pinot and the Reserve Chardonnay.
Medhurst - lovely setting and both owners were in attendance (Robyn running the tasting and Ross popping in briefly from the harvest). Really enjoyed their wines, and particularly their 50/50 Shiraz Pinot blend. They didn't offer their Reserve wines to try which was a shame.
Chandon - interesting, but a bit soulless. It was interesting to hear from our guide how much the arrival of Chandon had turned around the Yarra Valley's fortunes though. Tried their (new, I think) Sparkling Shiraz which was pretty average. Same price point as Majella's which I would much prefer. Was packed with Asian tourists which is quite the norm apparently!
Yering Station - as we walked up the drive I thought 'wow, it looks just like Xanadu'.. turns out they are owned by the same people. Anyway, had a delicious lunch here in their spectacular dining room. The tasting was great as they poured all their wines generously. I'd even made friends with the women in the group who didn't like red wine(!) and snaffled all of her Reserve tastings. The Reserve Cab Sav was amazing, as was their Reserve SV. The estate wines were all solid across the board, will definitely look to get hold of some of them in the future.
De Bortoli - cheese and wine tasting to finish the day. Tried their PHI 2013 Pinot from Lusatia Park which was really good. Their Sangiovese was pretty good for the price too. Really enjoyed their Black Noble which I've not had before. Very much in the PX style, think this has been in the graysonline 20% ebay sale before, so will grab some next time.
Really fabulous day, such a great spot.