Hi Ross
Yes... I had this many years ago..... there is a seal deep inside the box which can only be got by removing the box and splitting it. I was advised to change the tube as well. The tube was not expensive, but better replace it while the box is out rather than find it is a problem after.... clearly change the O ring as well.
The job is not difficult, but time consuming.
John
PS. These pics may help. Lip seal (No. 34 in exploded diag) is itemised on pic 1.
Apparently, all these seals gave trouble in the first few years of these Alps lives, but by the time I came to replace mine, a modified seal was produced....... had no trouble since.
1990 GTA Atmo, 2003 Jaguar X type 2.5SE Auto, 2018 Kia Picanto GT-Line-S 1.25