I have Lunar solar1997 had same problem with shower tap but no doubt yours will be different but I had to take out the whole sink unit to get to it the worst design I've come across any way to the point and they were very helpfull.
Lunar Preston 01772 337628 www.lunarcaravans.com
Check the website as the phone number May differ it's from my old records.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix caravans/motorhomes.