I've got spring clamps, can anyone advise me as to how to attack the rear springs, and what the removal/fitting procedure is please?

If, If I struggle - How much would a reasonable garage to change the springs?

You do not need spring compressors for the rear.

Jack the rear up, loosen the bottom shock absorber nuts both sides , place a jack under the wheels to take the weight off the suspension then fully remove the shock absorber bolts. Also take the two 10mm spanner size nuts off the spring holders located under where the spring sits then slowly lower the jack until the spring can slide out.