Thanks Ian! The SBLs had broken seals and, to my pleasant surprise, washing up liquid had been used prior to sealing them. The silicone came away relatively easily from the middle box (a bit of gentle application from the edge of a new Stanley blade removed the remainder). The 'gasket' was made of strips of self adhesive neoprene tape arranged on each side of the lower box opening. I pulled these off and used furniture polish to soften and remove the remnants of adhesive.
A few sheets of neoprene had been included in the sale. I measured the opening in the lower box, measured and cut a square that was 20mm bigger than the opening. I cut the internal edges of the sheet to make a gasket that sat over the opening. A smear of washing up liquid and a steady 5-6mm bead of clear B&Q silicone was applied, the mid box was lowered into position. Once the sealant has set (24 hours) the speakers can be moved around with care. Kneel before them, lean the side of your head against the lower box, get your hands in a good position under the bottom of the steel frame, then move a bit at a time to the side, forwards and backwards. Make sure the spikes are level. Play some music....
After a lot of experimentation I have found the best position for them and everything has come together nicely: smooth, but detailed treble and a solid tuneful bass.
However, I've realised after looking at the Tom Tom website that the gaskets are rectangular. Hence the question asked above. I'll cut the new gaskets and fit them tomorrow, with pics at each stage.
Remember, the £120 is plus VAT, that is £144 for two pieces of cut neoprene, a small tube of silicone, eight pieces of 0.28mm aluminium sheet and sixteen staples (that you will probably never use!)
Here's a few links:
Neoprene:
http://rover.ebay.com/rover/1/710-5...0001&campid=5338728743&icep_item=121215824485
Aluminium sheet:
http://rover.ebay.com/rover/1/710-5...0001&campid=5338728743&icep_item=272029424701
Silicone:
http://www.diy.com/departments/diall-sanitary-transparent-sealant-310-ml/268959_BQ.prd
Veneer pins:
http://www.diy.com/departments/avf-veneer-pin-dia12-l20mm-125g-pack-of-611/250510_BQ.prd
For around £20 you can treat your SBLs to a new gasket and sealant (and have many more applications left over)... £144? I don't think so.