Lego whitening help

My son has the Lego deep space rocket set (60228) but unfortunately it has suffered sun damage from sitting in a south facing front window! The attached image are the offending pieces after about 8-10hrs in 6% HP, split over 2 days (4:6 ratio) in April UK sunshine (periodic cloud cover). I reused the same HP on day 2. There has definitely been progress - I would describe the pieces as partly amber prior to treatment. Is it worth trying again (with fresh HP?) on a much sunnier day, or is this...