PDES wrote:

I recently moved into a new house which had new lawns laid. Until recently,
the lawns had been cut religiously but on leaving the lawn for three weeks,
a great many brambles started to go through. I doubt whether lawn
weedkiller would be strong enough to cope with this invasion so does anyone
have any ideas as to how to eradicate the brambles without destroying the
lawn?

Keep cutting it regularly through the season and try a broadleaf
specific lawn weedkiller as well. Chances are the brambles could survive
either one in isolation but not an orchestrated attack.

Be sure to follow the instructions about when to apply and when not to
cut the lawn. It will make a lot of difference. You have to sap the
strength out of what were probably old established bramble roots
underneath where your lawn was laid. Lawn weedkiller is too fast to be
fully effective in a single shot but it will help check them back.

Whatever you do don't allow the brambles to grow unmolested. Hit them
alternate weeks with lawn weedkiller or lawnmower until dead. Spot
weeding may be more effective if the brambles are localised.

Regards,
Martin Brown