The welding filter is a successful experiment, but the exposure times are not good for the old Pentax, which really struggles with long exposures. So I relented and ordered the real deal from Park Cameras. It arrived on saturday, and I did go over to collect it (on the way back from collecting another belt for the mower). Have had an initial play, it is much less dense (perhaps not dense enough!?). Have had some initially good tests and will add a proper piece here soon. The first real chance to use it will be in the Seychelles!