If the brakes don't slow the car down then it's the front brakes that are shagged, since they provide most of the braking force. The rear brakes only stabilise the back really.
If the compensator is stuffed, you'll get extra brake pressure to the front so while you need to check it, it's not the complete problem.
Take the front calipers off.. clean (with de-greaser and emery cloth) the sliding pins and make sure the piston seal isn't leaking and that the piston moves easily.
Clean the discs with emery cloth until they are matt. If they have a lip on the rim, get them skimmed or replace them if the lip is pronounced.
Grease up the pins with white brake grease and refit the whole thing with new pads. Use a decent pad.. you can do worse than the regular Ferodo's sold in the Big H... but if you want a fast road pad, use the DS2500 variant.
Make sure the pads are a snug fit but not jammed into their recesses. Use a file on the ends of the pads to make them not jam.. but you don't want them rattling about either.
The rears should be the same... The biggest problem there is likely to be sticking piston leading to a rusty disc. You can get a seal kit to overhaul the calipers but if life's too short you can get exchange calipers from Budweg etc... it's a total refurb using second-hand caliper case but with all new moving parts, bleed screws and seals etc.
The compensator sits under the car around the rear passenger footwell, on the nearside. The arm should move when you pull down on it (use a loop made out of a big cable-tie). You can take it off and re-grease it all.. but my view is that you only change them every 15 years.. so stick a new one in there since you're going to all that trouble.
I'd put new handbrake cables on the beast too.. just because yours will be knackered old 16yo ones..
Bleed the whole system starting with the caliper furthest away from the master cylinder.. and then Robert should be your mother's brother (at least once the new pads are bedded in).
One other thought... I dunno how much old 33 lives on in the 145 Boxer. Someone on here will know. .. but the 33 had shonky brakes too since the same little calipers that were fitted to the 1.2 Alfasud weighing 850kg,were carried forward more or less unchanged to the ton-and-a-bit of 1.7 16v (even the 137bhp 16v had to make do.. )
So the 145 Boxer (if it has 33 kit) may just have sh*t brakes...
It's still worth giving them a make-over though so that they are as good as they can possibly be... but I hope the Boxer had new brakes as fitted to the later T/Sparks.. since those are not too shabby at all.