I know the answer to this now - apparently it is 4 kilos of Science diet! Whilst dh was dropping ds2 and I at choir, our lab got into the cupboard where the brand new sack of dog food was stored, tore open the top, and opened the self-service department.
She went and pooed in the dining room (presumably to make space for more food), and when dh got back from dropping us at choir, ddog had to make a dash for the back garden, where she was sick. Several times.
Dh was worried about her, until he noticed the open cupboard and the torn sack - and saw how barrel-shaped the dog was! Her tummy was so full that she was sitting down on the lawn waiting to be sick again - she couldn't manage to stand!
I suppose we should be glad that it was the light science diet - we had thought she had put on an extra kilo or so, and wanted to get her back to her healthiest weight - but God alone knows what 4KG of food is going to do to her waistline (it's non-existant at the moment)!
Dh rang up his brother, who was with MIL in hospital (she's having major surgery tomorrow for a pancreatic tumour), and MIL laughed so much she nearly fell out of bed - so the dog has cheered her up, at least.
She is now lying in her bed, looking like she's expecting a huge litter of puppies, and feeling rather sorry for herself (the dog, that is, not MIL).