My Retirement/extension to stay O visa, runs out in April. I need to go back to uk asap till maybe Sept. Will I loose my visa and need to re-aply on return ? Thank you for any advise.

OP, I assume your finances are in place. Money in bank method?

What date in April does it expire.

You can approach immigration with evidence of departure and ask if it can be done earlier.

Which immigration office. 

If not you will need to start over.

Easiest path being return visa exempt and obtain non O and extension.

If you get new extension remember to get a re-entry permit before you go or back to start.

OP, just had second read of your post and notice return could be as late as Sept.

Not silly to just have it expire and return visa exempt and obtain new non O followed by extension.

Obviously would need to plan to meet the financial requirements.