Out of interest, what current is a small LED light likely to draw?

Now i am probably not the best person to advise on this especially after my post yesterday! but if you are using standard Light fittings like down lighters you get LED bulbs that are as low as 1.2 Watts (surface mount LED’s). P=I*V -> I=P/V -> 1.2/12 = 0.1 Amps/hr per light.

I would never suggest pinching a marlec turbine and mini pv array from an illuminated sign as they tend to also have the inverter & battery, try and find one 2nd hand?

Charging form your alternator is ineffective beacuse an alternator is designed to respond to load and not to charge a battery – it assumes the battery is charged and increases output when you say put on the lights for example

Bit wrong there. A standard alternator is designed to be a fixed voltage power supply, and whether the battery charges depends on the battery itself. An alternator is designed to put out as near it’s rated voltage as possible (typically 14.4V for a standard system under normal conditions), which is done by varying the current it puts out.
How much current the battery will accept, and how quick it’ll do that depends on the battery’s internal resistance, which varies due to type, size and condition. Certain batteries once discharged will develop a higher internal resistance than others, and take a period of time before the internal resistance drops enough for the battery to accept a proper charge.

It will charge at circa 50% its rated watts at circa 2000 rpm for about 10 minutes then the surface temp and other stuff will make th ealternator thingk the battery is fully charged and oit will trickle at about 5 amps [ this is irrespective of the size of the alternator or the battery bank .

Alternators only output there peak current/power under certain conditions, and unless spinning at a reasonable speed will reduce the regulated voltage/output as they’re internal temperature increases. This isn’t to protect the alternator, it’s a design feature to increase system voltage in colder weather.

or buy one of these
http://www.sterling-power.com/products-altreg.htm
which will trick the alternator into producing full power

Bit overkill, plus they open a whole new can of worms.
They don’t ‘trick the alternator’, they simply bypass the alternators built in regulator, to produce a voltage/current curve more suitable for charging batteries, but still not ideal.
These types of things when working are fine, but from experience on road vehicles, they cause far more problems than what they solved, and tend to kill alternators far quicker. On the vans I deal with, alternator life is reduced to about 20-25% of identical vans running a more conventional split charge set-up.

Personally, unless you’re really trying to preserve batteries long term, then just run a normal split charge relay (which in itself is just a glorified standard relay). Many will tell you this system isn’t ideal, but it’s worked for the past 50 years, and provided you size the wiring/relay right, it’ll work well enough for leisure purposes.
If you really want to preserve battery life, then get a battery to battery charge, as that way you’re not affecting the vehicle system other than a higher load on the alternator, and you get a charge cycle more suited for battery testing.

As for the type of battery. Look at semi-traction batteries. They’re more tolerant of deep discharge than a vehicle/normal leisure battery, and will start to accept a charge quicker than a full traction battery will from a standard alternator. Off course, you’ve got to weigh up costs. It’s normally cheaper to replace a standard battery every couple of years, than it is a semi-traction every few years…
Oh, and avoid gel batteries. Once they get discharged below a certain point, they need a constant current supply capable of 20+ volts to force them to accept a charge again.