Thanks for the additional information. It is helpful.
Many things can cause gas. It is believed that dog foods containing soy products and other food allergies contribute to gas problems as well. Sometimes a change in diet will help the problem. Often it does indicate a bit of gastrointestinal issues which might also lead to less than firm stools.
Smaller meals spread out can help and taking your dog for walks after meals cuts down on gas. Adding good bacteria to your dog's stomach can aid in the digestion as well. I use a spoonful of plain unflavored yogurt on my dog's food to keep good bacteria in their stomach, but there are products, such as Prozyme or Fortiflora, that can be purchased at your pet store to aid digestion and cut down on gas. I imagine similar products are in Scotland. This should aid in digestion and cut down on loose stools in the evenings.
Here is a site that discusses the problem:
Your dog is elderly and organ dysfunction can lead to loose stools as well. Parasites also can lead to loose stools as can protozoan infections such as giardia and coccidia. Read about these on the following page.
The first step will be to get a fecal test done. If you collect a fresh sample and deliver it to your vet they should be able to run a fecal test without an exam to determine if that is an issue. An exam is needed to have blood work done which might be a good idea given his age. Protozoan infection can lead to cycles of loose stools like you describe.
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