Pond

Hi I'm new to this group. I've had a pond form many years but had it made bigger 4 years ago. It wasn't dug as deep as I asked and now I'm looking at having it made deeper for my koi. I only have 4 Koi and other fish but want them to be happy. Thought 1.2 metres would be sufficient. WOULD that be deep enough?Any recommendations and what's the best time in the UK to do it as we are having massive heatwave at present? Was quoted £4 ,000 to £12,000 to have it made bigger!