The Ultracarb looks very brown.

I have to clean the Ultracarb every week. Far too often. I have tried 1um pleated and other prefilters but does not help much. What can I do to increase the time between cleaning?

How much water are filtering per week and how dirty is your water.

For normal household using the filtered water for drinking and cooking typical cleaning frequency is 2-3 months. If you are typical user and still have to clean it once a week then there is something wrong somewhere.

What is on the surface of the filter when you need to clean it? Does it feel slippery/slimy? If so then you have excess amount of heterotrophic bacteria (harmless) in your water supply which 1 micron pre-filter will not stop them at all.

We use about 10 litres (approx 2.5 gal) per day. The water appears clear. A lab report says that all is OK (E.Coli, mineral content and hardness etc) EXCEPT that there is a large amount of total coliforms (580 MPN/100ml).

The ultracarb looks very brown. The plastic ends in particular feel a bit slimy so it could be heterotropic bacteria as you suggest.
I will have a good look at it when I next have it out.
Could the bacteria grow on the outside of the ceramic, or will the silver content stop this?
Maybe I should use chlorine dioxide or hydrogen peroxide to disinfect the tank. Perhaps a 0.5um carbon block filter used as a prefilter may help a bit?

It is the bacteria that plug up the filter. No pre-filter can help you unless you use 0.5 micron absolute which you'll be back at square one, change out the absolute filter once a week and they are expensive. The silver in the ceramic controls the growth of bacteria from proliferating on its surface.

When you clean the candle make sure you scrub off until the candle become white. Remember that recommended change out is every 6-9 months or maximum 50 cleanings.