Two leisure batteries

Don't rely know, a bit of welder cable perhaps???

Make sure you wire them parallel or you will have 24 volts

Positive to Positive and negative to negative

Got me worried now when I added the second battery I used the cables from a set of heavy duty jumps leads are you sayings these are not man enough

David, good heavy duty jump leads are usually a similar diameter to those that come from the alternator to the battery.
Like myself David you may be able to remember the fuse wire we used to replace blown fuses in the home. 5 amp fuse wire was extremely thin and even 30 amp fuse wire was not much thicker than a few strands of hair. When it used to go regularly you may have been tempted to increase it to the next size up or string a few strands together. Eventually you might end up replacing it with tin foil or the golden foil from 20 benson and hedges. resulting in a "letting the smoke out" in either a melted cable,melted fuse box or melted socket internals if not a wee fire.
If you look at your vehicle fuses 12 volt, especially the 30 amp fuse it looks like a solid bar (in comparison to the 30 amp 240 volt fuse wire in the home), that has to melt in a short circuit situation,
Fuses in the home have been replaced with mcb's (miniture circuit breakers) to prevent people from oversizing the fusewire. Fuses are thermal protection if the circuits overheat due to the resistance of drawing to many amps in too small a cable or too big a load being put on a usually adequate circuit

Thanks for all that Guys, great advice there. I'll head for the motor shop this morning and suggest alternator/battery type leads then. Thenks again , Simon.

Was in the motor shop in Portadpwn yesterday and a guy bought a four foot length of suitable cable for £4. I reckon two of those would be ideal. He was going use the cable to jump start a motorbike apparently.

Get proper batt cable and proper connectors which should be soldered onto the end of the cable and heat shrink covering the soildered ends jumplead cable is no good for batt connections as its only designed or quick startups ( Ie) if you've jumped to cars you wil notice that the jump cables heat up ,don't use them as connectors between to batts get the right cable

They don't or didn't the last time I tried. Both the Varna and Bosch ones in that article are not easily got over here and those that can be found are expensive.

As another deviation, A&N caravan services have closed down which is a great pity as they are the only place I could find who repaired Calira charging units.