Here is the amended drawing with the missing measurements. Please use this in conjunction with the drawing in Garden Rail Magazine 253.
Garden Rail magazine No 256 has Jack part 7 with details of the chimney for scratch builders. Brian has added the following note to clarify:
For a 10mm ID Chimney you need the SCGP11 Summerlands Chuffer
For an 8mm ID Chimney you need an SCGP6
If in doubt, please ask!
Garden Rail 258
The handrails and rear posts are shown in the drawings as 2.5mm OD, but in the text Brian says that 5/32 brass tube can be used instead. Please note that this should be 3/32"!