Your leaseholder has to pay your reasonable legal and valuation costs (although not the cost of negotiation by your valuer) and is legally liable for your costs as soon as s/he serves you with a notice claiming an extension. The first thing your solicitor should do is to demand from the leaseholder a deposit of 10% of the proposed premium (or £250, whichever is the greater). Solicitors will have their own policies but they may be prepared to wait until completion of the new lease and take their costs from the final sum paid.