Leaking water taps (unusual design) ?

sink mixer taps (maybe FRANKE) leaking. Taps, stubby triangular shape about 25 years old in good chrome, how do I get to the guts eg past the nylon nonscrew insert below the coloured inserts (now removed)? Do I have to replace the assembly or just the sealing washer/o-rings? Help, my wife thinks my DIY skills are fading!!