Should I be worrying about this? The car is 12 years old.

Planning on getting the wire brush and waxoyl out to it before summer is over but will it make much of a difference?

Having seen some, yours looks to be in reasonable condition.
I clean and spray mine every six months, using a 50/50 mix of Waxoil and old engine oil. Mine's a 2009 Multijet, and the beam axle is still sound.

Make sure to keep the drain hole in each coil spring seat clear.

There are several threads on here detailing an axle change if you ever get to that stage - but avoidance is better than change.

That just looks like some gentle surface rust from the photos.
Unless you are seeing the metalwork coming apart in layers, or any sign of a stress crack forming where the spring cups change profile (the "squarer" bits) then simple preventative maintenance should be fine, as Sweetsixteen said - they dont look too bad at all in the photos.
Seen older ones better and newer ones worse.
Yet to actually inspect a failed one, and only seen pictures of a couple - from what I could see in those pictures it seemed to probably have been caused by salty mud accumulating in the spring cups and rotting them out.
Making sure the drain holes are clear, and washing them out with a hose (not a pressure washer) occasionally after winter might be a good plan.

What are some good ways to get the grit / surface rust off, is a wire brush as good as I can expect to get it? Seems like it would be hard to get in to all the important areas.

Chock the front wheels and jack up the car. Support on axle stands under the inside rear jacking point sills. Use some 2 x 1 timber to spread the load to axle stand tops.

Remove the shock absorber bottom bolts and the axle will swing down. You can now check inside the rear spring pans especially at the front just inboard of the welds. Clean out loose rust and seal with Waxoyl.

SB1500 said:

Should I be worrying about this? The car is 12 years old.

Planning on getting the wire brush and waxoyl out to it before summer is over but will it make much of a difference?

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same problem here, BUT id rather have yours! I got a wire brush attachment that fits onto a drill which made getting rid of the rust a lot easier! I think my worst crime was to then paint the whole axle beam and coil spring cups with hamarite. inside the cups as well. It don't look pretty but should stave off any more rusting or at least slow it right down! This job was done when conditions were dry and warm and the axle was bone dry so no trapped moisture.
Last years mot advisory mentions the rusty spring cups but , and I know he shouldn't have, he gave it a few whacks with a small hammer and declared it sound. better safe than sorry I guess.
Did have the axle waxoiled last year which has saved the axle from any more deterioration but that had mostly been washed away hence me giving it the paint treatment
On saying that, have I done a good thing ? seemed a good idea at the time. lol

hi, did you take any shots of the underneath of the coil cups you can show us?