Within 24 hours?

Within 24 hours?.

If i start t say 06:00 on a Monday and do a 9 hour drive finishing at 18:00 with breaks and poa filling the rest of it, and then have 10 hours off my boss wants me to start at 04:00 the next day and drive almost straight away .

Up to 06:00 the next day i will have done 11 hours driving in a 24 hour period is this legal or not?.

yes jammy you can start at 0400 no problem as you will have had 9 hours continuous break

The EC Regs define a driving period as being between two successive daily rest periods or a weekly rest period and the following daily rest period. If you started duty at 06:00 Monday then you need to end your day by either 19:00 or 21:00 to complete a daily rest or a reduced daily rest within 24 hour of starting duty on Monday.
You say that you end Monday at 18:00 and start Tuesday at 04:00. This gives you 10 hours daily rest (a reduced daily rest period) which means you are starting a new driving period and the two portions of driving would not be added together. This is why VOSA and the Police add driving periods together if you have had insufficient daily rest. They will keep adding the driving time until you have sufficient daily rest. You end up facing 2 charges; one of insufficient daily rest, the other of exceeding the driving limit between rest periods/