Sorry everyone, a bit abrupt, hi all, I have a voltage of approx 25v ac with all LED lamps removed, enough to switch on an LED

Live to Neutral

That sort of voltage is perfectly possible due to capacitive coupling, especially with 2-way lighting. What current flows when the circuit is off? (Use fused probes.)

Thanks Chris, I suspected that may be a reason, why would the RCBO trip when power removed from circuit

Is the polarity correct throughout? If the switch was in the N for instance opening the switch would mean that the wiring between the switch and lamp would be held at 230-ish volts - whereas it would be close to zero (neutral potential) with the switch closed. A high impedance fault to earth on that wiring (e.g. due to condensation or corrosion) would then leak more current to earth while the switch was open - which could be the cause of the RCD tripping.

   - Andy.

is it a two-way switch? One of the strappers might be a bit leaky to earth

No comes off a quick wire unit switch and load, so bog standard one way switch, the circuit has been working for 6 months, just decided to play up in this unusual way, as I say once the circuit is energised, see above, it will stay on continually until, I switch off and then the RCBO trips.

Does it still trip with the lamps taken out? It may be a scratchy light switch - some RCD/RCBO do not appreciate multiple sharp edges, better ones filter that out. 

can you clarify "can only reset by closing light switch" do you mean with switch in 'off' position it never resets? If so, a very long shot, but for the 2 mins it takes, just  take the switch off and see what is behind- at least one thoughtless decorator used the 'spare' terminal of a 2 way switch wired as a single way, as a handy terminal  to park a flapping cpc. Maybe he has visited there as well...
Mike.

It doesn’t trip with the lamps out

One of the lamps is most likely faulty by the sounds of it - I had similar and found a faulty LED GU10 out of the 6 was faulty.