Mine looks similar now, although due to the flow rate etc, decided to do the filter first. Pump picks up from a large filter chamber (3 in a row), goes throw sand filter, then splits (t-piece), into 2x 55W and then joins again (T-piece) before going into pond.
Played around with a flow calculator yesterday, flow rate is half if you double the pipe diameter (now 2x 50mm)
Added the second UV this morning, hope to see a significant difference when I get home.
By the way cheapest and quieckest was Wilpet in Kempton Park for a 55W light.