Put engine flush through a 135k mileage CVH engine and you will have problems for ever more.

Oil change with 5w30, run for a day or so and refill with 10w40 (new filter both times) would be better than a flushing additive.

I would just say to change the belt, water pump (runs off the belt), full service items and a set of the modified stem seals is always a good idea. That can be done with the head on.

If you are down Kent way Chris, you could always pop by my place and I'd sort the lot out for you one weekend (especially if you are doing the Kent meet on the cabriolet club in April? )