For those looking for refunds, here is a message recently posted on the Marriott news feed on their website.

Marriott Responds to Volcanic Eruption in Iceland

We are currently waiving cancellation fees through 21 April, for guests holding existing reservations at all Marriott branded hotels and Ritz-Carlton hotels in the United Kingdom and select hotels elsewhere in Europe. We are assessing the policy in other affected markets around the world on a case-by-case and daily basis. Guests with pre-paid reservations and groups are asked to contact the hotel directly where they have reservations to discuss the options available to them.