Been up in the loft tonight to get the crimbo tree and decorations down. They were a bit damp. I can't see exactly where the water has been coming in but overall the felt is in a bad state. I can't afford to have the tiles stripped off and have the felt replaced. Is there anything d.i.y I can do from the loft space to sort this problem out. Cheers.

If the felt is in a bad state I would have thought you already have reasonable ventilation just through the holes.  This is a serious comment as older roofs have no felt at all and these rarely had condensation problems.

If it's rain getting in this should be a localised leak and relatively easy to repair.

However if it is condensation you will need to increase the ventilation.  As woody says you could fit eaves vents over the fascias (I think that's what he meant when he said bargeboards!) or else vent through the soffit with one of the many soffit vent systems available.

If there are gable ends in the loft it can be helpful to fit airbricks at either end as this helps cross ventilation.  I would do this with a diamond core drill and fit circular air bricks.  These take less than an hour each to install.

Are you sure your water tanks are cold?  This may seem an odd question but I have come across many situations where systems are "drawing down and pumping over".  In other words the expansion pipe into the tank (usually the small central heating header tank) is pumping hot water into the tank and drawing it back down the feed.  In other words the tank is acting like a radiator.  This gives off lots of condensation!

Check that the end of the expansion pipe hasn't dropped into the water as this will definately cause this problem.

I have also come across systems where two cold feeds are taken to two hot cylinders but the cylinders are linked and this creates a circuit for the water to circulate on gravity, causing the water tank to get warm and give off loads of condensation.

So just check that both water tanks in the loft are stone cold, they both should be all the time.

Also make sure any shower fans or bathroom fans don't simply vent into the loft space as this warm, moist air quickly condenses in the cold loft.  All fans should be ducted to outside.

So basically the roofing felt is not there to stop the rain getting in, the tiles do that.  The felt is there to stop draughts, dust and debris getting into the loft space.  If the tiling is 100% then the roof cannot leak.  It's a mistake to try and add felt underneath if it's just to redirect rainwater that's getting in.