On vessels over 50m, the two white masthead lights (which I believe are called Range Lights), the forward one is lower than the rear one.
Do they just indicate it's a big long boat or Do they serve any purpose of somehow being able to work out how far away the vessel is?
cheers

They indicate a "big long boat".

I don't see how they could be used to determine it's range. This link has valuable info:- www.otenmaritime.com/international-collision-regulations/part-c---lights-and-shapes/application-definition-visibility-of-lights

I don't have a lot of night experience but what I have found is that when I have come across large vessels is that they have so many lights on it was impossible to discern exactly which were deck lights, step lights, crane lights, nav lights etc etc. The only way I was able to work out exactly what was going on was to refer to ship tracker.
In one instance it was actually 3 vessels, a ship, a freighter and a customs vessel all side by side and impossible to determine their nav lights past their brighter deck lights.
There's no way I'm getting radar so I just assume I'm invisible and give as wide a berth as I can and check vessel tracker.

I have found the same thing slammin, if they are doing some kind of work on deck and have their deck lights on, they are lit up like Christmas trees and it's a bit hard to get their aspect in relation to you.
But the other night it was a near full moon (which makes such a huge difference in the enjoyment of a night sail to me, I nearly needed my sunnies), the ships were clearly visible and easy to see which way they were going and I got a good chance to have a look at their lights. With the different height masthead lights making it even easier to get their aspect.
I then read in The Complete Sailor by David Seidman, these masthead lights are called Range lights.
And then reading this in The Australian Boating Manual by Dick Gandy:
pg 216
2: vertical positioning and spacing of lights
a) on a power driven vessel of 20m or more masthead lights should be placed as follows,
ii) when two masthead lights are carried the after one shall be at least 4.5m vertically higher than the forward one.
b) The vertical separation of masthead lights of power driven vessels shall be such that in all normal conditions of trim the after light will be seen over and separate from the forward light at a distance of 1000m from the stem when viewed from sea level.

So b) is what got me thinking maybe there is something very handy to know that I should learn, to work out how far away they are, but as you say you'd need to know the exact distance between them to work it out.
Still extremely useful to get their aspect, thanks all.
Good link Cisco.
cheers

Range lights are simply indicating that the vessel is over the 50m in length.

A tanker/bulky will usually have one fwd and the other set on top of the bridge ..... often hundred + metres separation, where as we have one fwd on the bridge and the other is up the mast ..... correct vertical separation but only maybe 15mtres horizontal separation.

Difficult for us to have anymore due to the "working" deck down aft.

Lights should not be affected by the working deck lights unless you looking at vessel from aft of abeam. In out case the stern light disappears amongst the working deck lights.

Best bet, use the VHF and tell them you are ????? in relation to them or use the AIS.