No doubt you have all proceeded well beyond this with your GBL Mallard tenders, but I actually did a little modelling yesterday afternoon!
Following a helpful lead from Saint Johnstoun, I purchased some lead sheet to make weights for the tenders. Cutting it to size with wallpapers scissors, (the best use for them as I hate wallpapering or any form of decorating for that matter), was easy, but two thicknesses were needed, and some means of fixing them.
As you can see, the Hornby tender chassis has two conveniently located screw fixings through the chassis bottom, and the posts these screws go into, have sufficient unused (but threaded) height, to down screw into, through the lead sheet, from the top. Washers were needed to support the screw heads.
Screws between 1st and 2nd and 3rd and 4th axles.
Holes drilled through lead sheet must be wide enough for the post to stick through, necessitating the washer mentioned above.
To secure the body to the chassis, I found that one of the plastic mounting blocks that came with the GBL model (as suggested elsewhere) did indeed work well as a post to screw up into.
Flat base of mounting block glued to underside of water filler area. Initially I tried super-gluing a nut to the top of the post, but found that the glue joint broke when the screw was tightened. It turned out that a nut was superfluous as a long screw made its own thread in the plastic.
Hope this is of use to someone!