Ok here's how it should work...
Currently the Bosch Rotal 36 Ergoflex is already on offer @B&Q for £89 (down from £129.98) which in itself is a good price, however this weekend B&Q are doing a 20% of a £100 Spend.
So if you add ANY item for £11 or more to the £89 Bosch Lawnmower, to make up the value of £100, you will then get 20% off.
In the end you will have the Lawnmower £9 cheaper than it is now + your £11 for free. Thus £80.
Also, for even an better saving, so via Quidco/TCB for 3.5% Reserve and Collect Cashback...
Hope that makes sense :-)
EDIT: Confirmation of 20% at B&Q below: