Answers for a new member from a new member
Gavin - I'll see if I can shed a little light.
There are several different bottlings you will see. First, the distillery's official bottling(s). As many distillers don't have their own bottling lines, their official bottling(s) may or may not be bottled at their distillery. There are many bottling companies in Scotland whose business it is to provide blending, and/or vatting, and bottling services for the distilleries and/or blenders who need them.
Second, you will also find distillers whom use bottlers, and those bottlers marketing arm, to be the sole bottler and branding arm for their official bottling(s). An example of this is Miltonduff - whose bottler is Gordon & Macphail, and whose official bottling is a 10 year. G&M is also the sole exporter and distributor of Miltonduff. This relationship may change over time. For example, do a little research on the history of Glenlivet, Macallan and Glen Grant and you will G&M to be involved with those distilleries official labels and bottlings for 60+ years prior to the modern era of single malts being kick-started (or re-kick-started) in 1964 with the release of Glenfiddich.
Third, the bottling you refer to, Port Ellen by Signatory Vintage is an example of a bottling by an "independent bottler." Other independent bottlers include Gordon & Macphail, Duncan Taylor, Cadenhead, Douglas Laing... I could give you more than 60 others. Anyway, some independents are better than others, and there are only a few whom are totally independent (G&M and SV being a couple who are). Meaning, they are not owned by a larger conglomerate or distillery.
Historically, the most significant is G&M. G&M buys new make spirit at cask fill and ages it themselves, and has done since starting their whisky program in 1896. They sometimes age at the distillery but usually in one of G&M's warehouses. A real kicker is that since they buy just a few casks from most of their distillers annually, they're more concerned with the quality of cask than most distillers are. Additionally, when they go to bottle, they are not marrying a 100 or more casks at a time like they do at most of the distilleries, so you're usually seeing a pured expression of the spirit in an independent bottling than you're seeing in most Glenmorangie, Macallan... official bottlings.
In fact, the independent bottler is more often releasing single cask, cask strength and non-chill-filtered whiskeys than the distilleries do. So, if a purer expression is what you're after, you will more often find it in independent bottlings than official bottlings.
Signatory Vintage is an outstanding company. Andrew Symington and Des McCagherty are long time industry experts prior to Andrew starting SV back in the late 1980s. Des came aboard when Andrew decided he wanted to try and buy Edradour (which did come to fruition, thankfully). Their good hands are involved in the selection of casks they buy. For example, Port Ellen, long closed, the only way to taste an example of their whisky is to try something from an independent bottler.
Buying bottlings from SV and G&M and a few of the other top independents is something you can do with confidence.
As for the independent bottlings versus official bottlings being more collectible: I would suggest the independents are generally more collectible. If they're releasing a single cask 13 year Macallan, there is only going to be a few hundred bottles released from that cask whereas the official Macallan 12 may be releasing tens of thousands of cases a year, not to mention Macallan doesn't release a 13 year. Of course, there are examples of official bottlings out there that will cost a small fortune - Bowmore Black and Cardhu vatted come to mind, if you can find them to collect.
A couple weeks ago I found a bottle of Dallas Dhu 10 Year, bottled by G&M. It was $40 U.S.! G&M tells me it was sent to the U.S. between 1993-95. Dallas Dhu was dismantled in 1983 by DCL, and is now a museum. Is it worth more than $40? Surely! But I bought it to drink.
I used to work in wine auctions years ago and we used to say there were two kinds of collectors: One who has the money to collect and one who has the money to drink. Everything I've bought has been with the intention to drink, even though I don't fall into the financial category of either type of collector.
Anyway, I hope this answers your questions somewhat.