I have to turn my shower to full temperature to get any heat out of it but when its on full the pressure is just trickle. I used to have it on number 5 but now number 5 just gives freezing cold water - any ideas?

Lots of shower valves have pressure balancing cartridges nowadays in order to get full flow out of hot or cold you have to have both the hot and cold hooked up to give even pressure on both sides of the cartridge that should fix your problem and give you a full flow

Newer showers use safety valves to keep people especially children from getting burned they are a single valve with three ports. One ,hot in, cold in, and one port for tub and shower. You need to pull the valve and look for tears in the valve seat.

If its a trickle occasionally its normal, when you turn off shower the water is still in pipe right up to shower head, it cools down, contracts and backs away from the head, when house gets warmer water expands up to the shower head and trickles out.

Hello Mark
is the water still coming out the bottom when you pull up the lever to make the shower come on ? if it is then the tub valve is bad . if little or no water come out of the bottom then the head is going to be clogged and you have have buildup inside the filter /screen which could be located in the end of the hose at the head or inside the head unit itself. .

It sounds like the numbers are for the temperature indication and the higher the temperature the more slowly the water runs because it takes longer to heat it.

Sounds like the filter on the hot water inlet is blocked. Is this a new shower or one that has been in use for some time ? Also , what make?

It sounds like it has mineral deposits in the hot water side. Try taking the internal valve parts completely out of the fixture and examine them. If there is any scaly (light grey) deposits, that would be the problem.
The solution is to soak all the parts in vinegar, which will dissolve the mineral deposits. It takes awhile to dissolve what has taken long to build up, so heat the vinegar, or put it all on a warm, not hot, surface to keep it more active.
I had this problem, and after cleaning it a couple of times with vinegar, I changed out the Delta to another brand. You could also use a water softening unit, which replace calcium and other metals with sodium, thus eliminating the mineral deposit problem.

Could the boiler be clogged with lime deposits such that the flow is restricted through the heating coils?

Hello, first if any water supply hot or cold is turned off, you will experience trickling as you mention. You must have both cold and hot in order to adjust the temperature on your valve. if you had a plumber intall this unit you should ask him to adjust it. Otherwise here is how:
Rotate the handle counterclockwise to stop and let water run till cold water is purged, check
temperature. Rotate handle to off, remove handle parts (see Disassembly in your manual) and temperature limit stop, rotate clockwise to decrease and counterclockwise to increase maximum hot water temperature, reinstall temperature limit stop and handle (TIP: Do not screw handle in until desired maximum temperature is achieved) Repeat steps until desired maximum
temperature is achieved

You could have mineral deposits on the temperature probe inside of the unit. If you remove the heater and purchase a bottle of lime and scale remover (lime away) to put inside of it. Let the unit sit full of water and chemicals for 24 hours. Flush out the unit to remove any residue. You may see the water is brown if you have a lot of buildup. Reconnect unit and test your settings. If this doesn't work, install a anti-scalding device, available at a plumbing supply store.