Have a look on DISCO3.CO.UK - Dedicated to Discovery 3 and LR3 Owners and Enthusiasts there area few who have waterproofed the LR RAI. You will need to replace the fabric hoe, fill the bend and seal the connection between the snorkel and the boot as well.
I made a removable one, see here it will show the bits you need to seal/replace.
DISCO3.CO.UK - View topic - Removable Snorkel