Metering is something that is a bit of a foreign world to me. The only real metering I do is LED Bars, VU Meters and EQ/Spectrum analysis to help me 'see' the sound of my music etc.
Is there anything else that is essential / highly useful to consider looking at when mixing? And if so does anyone have any recommendations for good products at reasonable prices? (I know that sentence is vague).

Beyond the tools you mentioned, the only "essential" meter missing is a loudness meter. SPAN can do K-metering, and that'll do if you don't want to spend any money.
 
However, k-metering and VU meters don't really take loudness perception into account very well, which is why EBU loudness meters have become popular. Even though its purpose is to set standards for broadcast, it's still helpful even for music that will never be heard on radio, TV or film. 
 
TB EBULoudness from Toneboosters is just 19.95 EUR, but Meldaproduction has a free one.

In the brief time I've had it, the T-RackS meter has opened my eyes to what I'm hearing and not hearing.

bitflipper
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TB EBULoudness from Toneboosters is just 19.95 EUR, but Meldaproduction has a free one.

Hey, bit, I've got both. I forgot about the Melda analyzer that came with the free bundle. Any comments comparing the two?

The Melda one is more CPU-intensive than the TB one, a minor concern given that you slap this kind of meter on at the end of your process and can bump up your buffers for mastering. Both give you essentially the same information, which any "EBU" meter must do by definition, although the Toneboosters plugin also displays K-metering. So you get a few more bells 'n whistles by paying for the meter over a freebie.
 
These days I'm using the iZotope loudness meter. I like that one because you can set your own target if you're not strictly going for official broadcast standards. Being able to size it big is appreciated, too.

Thanks.

Thanks everyone, I'll have to dig deeper into EBU meters and how they 'work.'
I'll check out all links thankyou.