My heater blower seems weak even when selected on max. Is this normal ?

They arent amazing power but should be noticable

When was the pollen filter last changed ? as mine was pathetic ,but after a new filter great improvement

As said above, pollen filter makes quite a difference if blocked, but having said that, the output to screen or footwell is no tornado, but if you select the centre face vents only, you should have enough air pressure to make you start rubbing your eyes.

I'm wondering if there is any point to putting a new(er) blower in. Has anyone done this ?

Definitely check the pollen filter - Did mine today and found it clagged to the nines with whatever was dropping leaves in the PO's front garden... sycamore, I think.

Needs a blow-out still, if not outright replacement, but even that has made a difference.

A charcoal one is £7.80 on carparts4less, fill your boots.

 

Should be replaced every 2 years really.