As the whole idea of these types of burial ground is to look as natural as possible, large monuments do not go down well with the whole ethos here. Each burial ground may have slightly different rules and regulations, but many will still allow the grave to be marked discreetly with a small wooden memorial if desired. Stone monuments or anything made from non-biodegradable materials will not usually be permitted.
Many people are concerned that without a physical marker, they will not find the grave if they wish to visit in the future. All reputable sites however, as a matter of course should have a detailed survey of the site layout so they can pinpoint exactly where all the graves are, not just for family and friends of the deceased, but also for their own information so old graves are never disturbed when digging new ones. Many will use very accurate digital surveying equipment, micro-chip data logging, or GPS (global positioning system) - just make sure (for your own piece of mind) that some form of accurate records are maintained.