There are lots of Arnolds in Isleham, Tom - just no James of the era you sugest that I was able to find.
However, I did find Susan Arnold's marriage to Henry Musk:
9 Nov 1843
Henry Musk, of full age, widower of Pound Lane Isleham, s/o John, lab
Susan Arnold, minor, of Pound Lane Isleham, d/o Diersley
Witnesses: Charles Westley and Geo Fletcher
So it looks as if your Susan Arnold is the one baptised 3 April 1826, d/o Diersley and Mary (Sorry - I said 1825 before, but I mistranscribed it!) Of course, she could have been born earlier - Derisley and Mary baptised James and Ann (possibly twins as they ran in the family) in 1821, and then no more till Susan in 1826, who appears to have been their last. So she could easily have been actually born in 1823.
Interestingly, when you look at the census returns, in 1841, Dazeley and Mary are in Pound Lane with James (a turfman) and Susan aged 20. In 1851, Dazely and Mary have moved to East Fen Drove, where James and Susan aged 30 and 32 plus four children are listed with them as the "lodger" and his wife and family - all born Isleham. James is listed as a Turf Cutter and Dazely as a Pauper. Cjhildren are William 10, Ann 7, Mary 5, Pament 1.
Pamment is interesting, as it is a very well established Isleham family name. James and Susan had two tries for a Pamment - baptised Mar 10 1848 (buried Sept 5) and Mar 17 1851 - so the name must have been important to them. Her maiden name, perhaps?
So back to the marriages in Isleham gives us:
Oct 12 1839
ARNOLD James, minor, bachelor of Pound Lane Isleham, s/o Deirsley lab
PAMMENT Susan minor, spinster of Church St Isleham, d/o of Thomas lab
Witnesses: William PAMMENT, Geo FLETCHER
And the baptism records DO give us this James, baptised 18 Dec 1821 s/o Derisly & Mary
So where was Susan, d/o Diersley and Mary, in 1841?
There is a Susan Arnold, living-in as a female servant aged 15, at Heath Farm in Chippenham, with farmer Thomas Kent and his wife Susanna.
And a Henry Musk, aged 21, living at Sounds Farm in Chippenham, with parents John and Allice and brother Joseph 15. There is a one-year-old baby in the house, Anne Maria. So perhaps Henry had already been widowed by then?
I found his baptism on the IGI - 07 Nov 1819, Cambridge, s/o John and Alice Musk.
So it looks as if you have confused some issues somewhere - your Susan who married Henry Musk is the daughter of Derisley Arnold and his second wife Mary Sanxster. There WAS a slim link between the Arnolds and the Cadmans - a Derisley Arnold was witness at the wedding of John Cadman and Susanna Pareman in 1789, and one of the several Derisley Arnolds and John Cadman were baptised on the same day in 1761, so probably grew up together. All the big Isleham families were heavily intermarried and closely related, like all Fen communities.
If you need any further help from the records, do come back to me - I have them on file.
Wendy