Very slow weight loss on OMAD

So it's been 6 ish weeks and you've lost 6 pounds? I don't think that's a very slow loss at all, 1-2lbs a week is considered healthy/sustainable weight loss, any more is into crash diet territory, so while you are at the lower end of that I'd be happy - depending on your weight it sounds like you don't have a huge amount to lose to get into a healthy weight bracket and so it's always going to be slower going. Also as you have found (healthy) weight loss tends not to go in a liner fashion, you can stay the same or even gain a little for a few weeks before seeing a big drop, all the in the context of an overall steady downwards trajectory. It could be that you are experiencing a bit of water retention or constipation or something and you'll suddenly drop a few more pounds...

So I wouldn't despair - that being said if you are doing the Fast800 as opposed to eating your normal TDEE/TDEE less a %, this is classed as a Very Low Calorie Diet so it's important to be eating the right kinds of food - the Fast800 should be a low carb Mediterranean diet. Not saying you've been eating crap but if you are only eating 800-1000 cals of 'normal' food and increasing exercise at the same time, that might be causing you some metabolic type issues? The Fast800 is quite difficult to sustain in the long term (I think you are only meant to do for 6-8 weeks) so are you planning to move onto regular 5:2, 16:8 or something else - confusingly you might actually find that the weight loss rate stays the same or even increases if you are eating a little more most days?

1lb a week is 4 stone in a year

Someone far more knowledgeable than me posted some evidence on one of the bootcamp threads that OMAD may not be terribly efficient for weight loss and splitting the intake could increase it.
Might be worth a look or ask the question over there

Have been doing OMAD ( 20-4) since May and have gone from size 16 to 8/10. It has become a lifestyle and I feel so comfortable and energised with the regime.
Some days I do 18-6, some days 22-2. If I get really hungry during fasting hours, I'll have a couple of boiled eggs and a handful of nuts. Mixing up the fasting hours and eating more during some days surprises the system and keeps it guessing, hence the weight loss.
1lb a week is good going.

The really surprising thing about fasting is that you can lose many many inches even if the scales don’t move. It sounds like you’re experiencing this.
How about doing your measurements aswell or instead of the scales?