> I am fostering a young dog of a herding/ Guarding breed. He is very reactive to lots of things including people dogs, cyclists etc
You've had great help re. the guarding already.
When Buster was about 8 months old, he suddenly found chasing my children around the garden, whilst they were riding their bikes, a very fun game. The first time he started this, he actually grabbed one of my boys by the seat of his trousers and dragged him off the bike

It was all very much play (my boys not helping matters by yelling in delight!), but it was obviously not a wanted behaviour and I was very worried it could quickly develope into something more than just play (chasing and bringing down a person on a bike, is far too hunting-like for me!!!).
I started to desensitse him by STOPPING my boys playing on their bikes in the garden and instead, coming with me & Buster on every dog walk. To start with they rode their bikes very slowly next to us. As he got used to this, I had them pick up a bit of speed, but further away from us. We worked up (all within Busters limits and never getting to the point he got excited), to them whizzing around at full speed, cutting infront of us, zooming up from behind us, when Buster was solid with that, I got my boys friends to join us. We ended up walking with a group of 5 boys (age range from 9-15yrs) riding thier bikes like loons around us, even doing jumps & wheelies around us.
Buster was rewarded by food and occasionaly a bit of a game with me (a bit of rough-housing as he doesn't 'do' toys).
It was only a matter of weeks before ANYBODY on a bike was of NO interest to him.
Over the next few months, I'd get my boys to come with us on thier bikes every now & then, untill they gave up thier bikes for scooters. Fast forward to now (5+ years later) and Buster has retained his learned attitude to bikes. My youngest son just had a bike for his birthday, came with us on a dog walk, Buster was so un-interested he didn't even notice my son had zoomed towards us and screeched to a halt infront of us - Bust actually walked into the bike as he was more interested in sniffing!
I used walk times for 2 reasons, 1) there is much more space out in the countryside than my garden so if distance from the trigger is needed, you have it available and 2) there are lots of distractions out on a walk (sights/sounds/smells) that make the trigger a little less triggering than if it were in the boring garden.
The same thing can be used for joggers, you'll need some volunteers to come with you on walks to jog with you :) I've not set this up to work on with Buster, I just move him off the path, have him stand still and feed him bits of cheese for standing calmly whenever we happen accross joggers, but he's never been then reactive too them anyway.
It really is worth going to the effort of working hard on this one. I'm so pleased I did. We've had a stranger zoom around a corner on a bike, from behind us (on a pavement

) and Buster didn't even flinch :)