Thanks all for the helpful advice. I managed to extract the washer bottle by removing the top neck which is under the trim panels under the bonnet (thanks Matthew - it had me stumped for a bit) The spout simply pulls out. Next was to remove the wheel arch liner (lots of bolts) which then allows access to the bolt holding the tank in place. I also removed the air duct at the back of the wheel arch thinking this was an obstruction but don’t think this was necessary. The tank tilts forward and then up to release the dog leg at the bottom of the tank. The pump is pulled out of the tank and you can clean the end filter. Mine was completely gunked (see picture) so a good clean with the wife’s toothbrush (kidding) sorted it out. Tank can be disconnected to wash it to remove the rest of the gunk. Refitted and filled with BMW screen wash. All working as it should now.
Thanks to all for the advice, hope this helps others if they have a similar issue.