Adhesive to adhere Hardibacker board to masonry wall

[DazJWood]

All,
I have a solid wall over my bath which originally had plaster skimmed over the blockwork. I want to install travertine but having read up on the weight restrictions I have now removed the plaster. The wall is now down to the blockwork.
I have contacted Hardie to ask for their recommendations for fixing Hardibacker board to the wall. My plan was to install Hardibacker 500 (12 mm) but they confirmed that unless I needed the depth that 250 (6mm) would be equally sufficient and would support the same weight. They also sent me through a pdf (see attached) detailing fixing with tile adhesive and mechanical fixings.
The pdf says :
"Use tile adhesive that complies with BS EN 13888:2002 or
preferably BS 5980:1980, type 1, 2 or 3. Adhesives for use
with ceramic tiles and mosaics."
Now I have Excel Bond Rapid Set Flexible adhesive for my travertine tiles and this has the BS 5980 number detailed on the back so I'm guessing this is fine to use. If I do use this though I am sure I will run out when I start my tiling. As this was expensive at £43 a bag I am wondering whether there is a cheaper tile adhesive I could use that will still conform to the requirements and adhere the tile backer board equally well.
I have looked round the internet and also been down to B&Q and Topps and must admit I'm a little confused by the different adhesives and the BAL and Mapei ones don't seem to quote any of the numbers listed in the pdf. I know these will be suitable so I would appreciate any guidance or if possible a link to a couple of products I could go on the hunt for.
Incidentally, what is the difference between an adhesive that is described as "Rapid Set Flexible" and "Single Part Flexible"? I have read up on both products (made by BAL) and they appear to do the same thing?
Thanks in advance,
Daz

[david campbell]

most bagged flexible adhesives will be fine for this,i would contact your local tile giant,you will get reasonable prices for mapei adhesive compared to bal probably about 33% cheaper

ps the difference is setting times,rapid setting usually has a pot life of about 40 mins and is totally set in 3hrs wheras if it doesn't state rapid setting then the pot life can vary from 2-8hrs and set from 4-24hrs

[DazJWood]

Thanks!

I have just checked the Tile Giant site:

Are these two adhesives suitable and if so what is the difference between them except price:

Keraflex Maxi Grey 20kg Flexible Floor Tile Adhesive - Wall Tile Adhesive - Tile Adhesive & Grout

Keraquick Grey 20kg Fast Setting Single Part Flexible Floor Tile Adhesive - Wall Tile Adhesive - Tile Adhesive & Grout

Thanks,

Daz

[david campbell]

very similar adhesives but the
Keraquick Grey 20kg Fast Setting Single Part Flexible Floor Tile Adhesive - Wall Tile Adhesive - Tile Adhesive & Grout
is the rapid setting