Pigletjohn, or anyone else who has had the same problem, I am hoping you can help. I bought an Alveus sink made in Slovenia (need a particular size sit -on sink to fit an existing unit). It was fitted in the summer but when it came to plumbing in a dishwasher recently there is no spigot to attach the waste hose to, just a blanking cap. I went to the plumbing suppliers but they had parts that were 1.5" or 1" and this seems to be roughly 1.25", a bit worried that there could be an issue with thread compatibility too. I have phoned the suppliers but no-one has asked then this before, they are going to get back to me. I have been having a lovely time reading the McAlpine plumbing products catalogue but don't want to trail out and come back with the wrong thing. My plumber is good but elusive and very busy at the moment so i decided to have a go myself.
I did that Pigletjohn, one size was too small, one too big, and nobody told me there was a size in-between. I was wondering if a coupling piece to change the size would work, I will try and put up a pic soon. The kit that came with the sink seems a bit flimsy compared to the sturdiness of the trap and waste my plumber put in so if it comes to the worst I could always replace the bits that came with the sink to all BSP parts.
Trying to find plumbing parts makes Christmas shopping seem like fun!
I would go for a stand-pipe and trap attached to the waste pipe then. The hooked end of the appliance waste pokes loosely down the pipe - it should not be sealed in. You will see them in the online cat. Use the full length of the pipe if you can, going nearly up to the underside of the worktop. It can be fitted to the waste pipe at any convenient point, it need not be under the sink.
You may (should) find that the condensate pipe from your boiler goes into the kitchen waste as well (this is preferable to running outside where it will freeze in cold weather)
If the soil-pipe is accessible in the corner of the kitchen, a stand-pipe or additional waste pipe can be fitted, but you had better get a plumber to do it.