Following the advice in the WiKi
https://wiki.batc.tv/Filter_Modulator_build and before I had receipt of an assembled unit, I purchased a Heatsink produced by Ohmite (Part No. FA-TO220-38E Farnell code No. 2097690). This TO220 38mm high heatsink was chosen for its low thermal resistance and appears to have the correct fan-end profile, but it is 25mm wide and fouls J41 'LO' SMA socket and headers J02 & J03.
To enable others to learn from my error, could I suggest that the wiki page is amended to indicate that
a TO220-38E 12.7mm (1/2") wide heatsiink will be required
the board not being designed to accept the wider 25mm (1") varieties.
Suitable types include Ohmite Part No. EA-TO220-38E (Farnell code No. 2097698), or AAVID THERMALLOY Part No. ML73/38 w pins (RS Stock No. 712-4254)