Search through the major UK online chandlers for prices. Lavac spares can be very expensive. I reduced a potential £400 bill to about £150 by buying various parts from different online chandlers. With Lavac stuff it pays to shop around. My loo is the Zenith which is now a redundant model.
Pull the seals out & clean them up. Only replace if they have really hardened with age. Hissing leaks can also occur at any of the joints, especially on the intake side where air is drawn in under the vacuum & no lquid escapes.
Also, make sure the inlet is clean & clear, lots of Barney Kulls or limescale/ corrosion partially blockingthe intake can lead high pressure & low flow. Oh, and don't forget that the vacuum release pin hole will hiss. Finally, don't kneel on the lid to create a seal, the seat or lid will probably crack & the replacement cost will make your eyes water.