Here some more info on the 535
http://www.shetlandboats.co.uk/shetland535.htm
Here's the 536's,
http://www.shetlandboats.co.uk/shetland536.htm
you will note although the 2 boats are similar in dimension, but they are infact totally different. The 535 is very light, single handed launch is no problems, whereas the 536 is more than 200kg heavier. The extra weigh is due to the 536 is a double skinned hull filled with poly..wotit, ie, unsinkable. However, the extra has push it over the 750kg mark and requires a braked trailer.
The hulls are also different, 535 has the same hull as sheltie (a V ) whereas the 536 is same as the family 4 or 4+2, a gull wing design (must be a good one, cos the Raider boat uses the same design).