Is it true that my flexible shower hose must not be able to reach into my WC or bidet?

Yes it is. Any shower hose that reaches into a WC or bidet bowl is a fluid category 5 risk of contamination to your drinking water supply. There are several ways to remove this problem, such as constraining the hose with a clip so that it cannot reach the WC or bidet, fitting a fixed shower screen or simply fitting a shorter hose. For further advice please visit the WRAS website.