Brittish Ale yeast question/suggestion

So I have made a few batches of a Surly Bender Clone. 1st batch I made 3 years ago and it turned out amazing and just like Bender. For that batch I used Wyeast London Ale III 1318.

So now I have to brew a number of batches of this same beer for my brother in laws wedding. I have brewed 4 batches so far and used the same recipe with the London ale 3, and have tasted and bottled 3 of the 4 batches so far. All of them have a very fruity/estery tasted that the first batch did not have. My fermentation temp is at 64* (taken in a thermowell in the fermentor in a temp controlled freezer) and I am OCD about sanitizing.

Is it possible that this yeast strain is not what it was 3 years ago? I am thinking of making the next few batches with either Brittish Ale II 1335, Brittish Ale 1098 or just going with the clean and neutral American Ale 1056. I originally used the 1318 as it was recommended by Northern Brewer for their Bender clone.

What yeast would give me the closest flavor to Bender and not have any fruity esters with it?

So I think I just had an epiphany...Tell me if I am on the right track.

I have been brewing these newer batches of the beer in a Brew Bucket from SSTech. It is a 7 gallon fermentor. I put 6 gallons in and leave 1 gallon of head space. On every brew, I have had a decent amount of beer and yeast come out of the blow off tube into the sanitizer bucket. Is it possible that once the yeast gets blown out, that then I have too little yeast in there to keep fermenting and it is stressing them out causing the ester flavors? When I rack to the bottling bucket, there still seems to be plenty of yeast in the trub, but maybe I am loosing a large amount in the blow off.

Should I drop down to a 5.5 or 5 gallon batch in the 7 gallon fermentor and leave more head space so I am not sending yeast out of the blow off? Right now I am using a 1/2 blow off tube. Would going to a 1" reduce what actually comes out of the blow off, or would the same amount of beer and yeast get expelled through there as well.