Hi Phil
No joy I'm afraid. I managed to find 30 minutes with no rain and explored as much as I could. I walked all Row R, and as I said there is no sign. The row is split by 2 paths - the very oldest section (1900 ish) is on the Southern end and quite overgrown, there is a section between the paths with only one or two headstones, and then the Northern section has graves starting at 1983 and running to 1986. If you took the oldest (southern) section as starting at number 1, then number 55 would be in the 1980s part, so it looks like an error. I also looked at any CWGC headstones I could find. These are mostly WW1 and just after (there are 4 in one row from 1919 - presumably the flu epidemic), but I found a pilot from October 1940 (in about Row G, but mixed up with more recent graves). I think your next step might be to ask Poole Council. There is a notice at the entrance referring enquiries to Poole Crematorium (01202 602582). They presumably hold the records, and may be prepared to confirm the location, and if it is correct, where the numbering starts, and which bit of Row R the grave is in. I can't believe it doesn't have a stone, as the CWGC would have sorted that, unless there was a family stone.
The only other possibility is - are you sure it is Parkstone Cemetery, and not Poole or Branksome?
If you find any more, let me know and I will have another look.
Richard