I like wallowing in a deep bath but the only suggestions offered for increasing the depth required blanking the overflow and losing its valuable function. I tried twisting the elbow at the top of the flexible overflow pipe to point about 45deg upward, making the flexible pipe loop up before descending to the trap. This caused the overflow action to fail completely, so I pierced a small hole through the flexible overflow pipe at the top of the loop, to prevent airlock. The overflow action is now fully effective and no water ever emerges from the pierced hole. The arrangement behaves as though the overflow grille had been raised by about 40 mm and the extra bath water depth is a delight. Does this arrangement violate any regulations or codes of practice?
The overflow (or the bend in the pipe as you have placed it) must not be higher than the edge of the bath. This is to protect your home. The overflow is placed where it is to allow for the rise in wayer level as you get in. Raisng the overflow pipe is a dangerous thing to do as water cannot run up-hill. See diagramme below.
Thank you, thedoctor, for your interest and advice. The modification raised the overflow level by about 3 cm with taps closed and by about a further 3 cm to about 2 cm below the bath top with both taps fully open and giving a total flow rate of about 15 l/min (low head in my flat). The top of the pipe bend rests against the underside of the bath top. Until the small hole was made at the top of the bend, the bath could flood due to air lock.